I've read a lot of oil change questions about mileage intervals both here and on other forums. I can't seem to find the right answer to this....

I don't put a lot of miles on my '04 RX8 (it's got just over 14K on it now). I bought it new but it's really my second car so it doesn't get a lot of miles put on it. So, I can go 5-7 months before I put 3000 miles on the car. Should I be changing the oil at 3 months, regardless of the miles, or is it ok to wait until the mileage is closer to what it's supposed to be before I change it?

 

i dont think it's going to go bad on you, i would think just waiting until 3k will be sufficient.
There are some serious oil guru's on here that may know more, but i think youre good.

 

In your case change it every 6 months. u're still good

 

The manual has time versus mileage charts, specifying to change oil whichever comes first. That means according to the driving conditions referenced to the correct maintenance schedule for those driving conditions, oil is changed when the specified miles are reached or the specified months are reached, whichever comes first.

This is all from memory and I am hours away from my manual right now.

Personally I will change oil at 6 months if I have not reached 3000 miles.

 

just as with my other car i actually car about, 6 months is always the best deal if you don't get to 3K miles first. In most cases, I almost aways max out the months first on the Cobra, but am lucky to even make it 1/2 that time with the 8.

Although it's a rotary, oil intervals are surprisingly close to many piston engines.

 

I remember a "oil guru" telling me that it is ideal to change at roughly 3 months because the oil molecules have a tendancy to breakdown over time. Perhaps a "oil guru" will see this post and comment to confirm.

 

Schedule 1 maintenance calls for changing the oil every 7500 miles or 6 months, whichever comes first.

Schedule 2 (severe service - which is typical urban/suburban driving) calls for 5000 miles or 4 months.

Personally, I've been in the habit of changing oil on my cars every 3000-4000 miles, and at least every six months for the lower mileage ones. I can't back that up with any kind of expertese, though.

Ken

 

I've always found the time-based logic confusing. If the time-based interval of an oil change is dependent on the time-based deterioration of oil molecules, the fact that Schedule 1 and Schedule 2 have different times (6 mos. vs 4 mos.) confounds me. Unless I'm missing something, they should be about the same, no?

 

I don't think it's deterioration of the molecules. If it was, new oil wouldn't have much of a shelf life. You change oil because it gets dirty. Under severe service, more crud gets into the oil, so it needs to be changed more often. Either by miles or time.

I suspect that if you never run an engine, you don't need to change the oil.

There's something I wonder about for rotaries. In a piston engine, most of the crud comes from blow-by of combustion prodicts, with a good part of that (maybe most?) coming from starting and from running cold before it warms up. After it warms, most of the crud boils off.

Where does the crud come from in a rotary? I didn't think there was blow-by to the same extent (if at all) as in a piston engine.

Ken

 

I wonder if whether the car is garaged or not would contribute to this time based oil change period . Logically I would suspect that cars left outside exposed to the elements 24 hrs/day would require oil changes more often .....

 

IMost oils nowadays have special additives that help extended the regular maintance of oil changes. But I would assume yes that they should be about the same.

 

Your logic seems impeccable.

My theory (which is mine) is that when they give you the time-based schedule, they are assuming a "normal" amount of driving. That is, they give a time-based version for people who don't pay much attention, don't remember what the mileage was last time they did the oil, don't notice that it's been enough miles for a change. You know: most people. (as opposed to some of the **** people around here). Much easier for the average person to handle "gee, it's been about 6 months since I had the oil changed."

I agree, if you're not putting down the miles, you shouldn't have to change the oil. I'd be interested in what rotarygod would have to say.

 

Oil dilution by fuel has to be taken in mind.

 

Yep, I agree with ya'll on this. But even the mileage numbers are formed around a lot of assumptions. If you leave the engine running and drive 0 miles, you'll still have to change the oil. The thing is, the margin of error can be, say, 50% and it still doesn't matter much. 5000 miles, 7500, 3500... ehh.

Oh, and about the car sitting in the garage vs. "exposed to the elements" - good one!

 

I was doing a search, and found a thread of the same question that you have. In there you will find various opinions of time vs mileage, etc. to see what other people are thinking.

"When do you rx8 dudes change your oil?"
https://www.rx8club.com/series-i-tech-garage-22/when-do-you-rx8-dudes-change-your-oil-74848/

RP
Even with RP, at 2500 miles and 6 months, running 2nd gear up into redline clearly isn't the same feel as when the oil was fresh. There is something happening to the oil even at sub 3000 miles: the sound, energy, feel, and vibration of the engine is not the same as the day oil was changed. I don't know what it is, but it's there and only a fresh oil change will fix it.

Dormant periods
In spite of this, last time I had the vehicle in storage for 6 months, upon taking it out of storage I drove it straight to the DIY shop to change oil and coolant.

More logic
I just think like this: Can't hurt it by changing too frequently, but you can hurt it by not changing frequently enough.

 

Unfortunately, there are market forces that have a vested interest in convincing vehicle owners to change their oil more often than necessary. The legal prey of these market forces have become convinced that they are purchasing "cheap insurance" or "peace of mind" by changing their oil more often than necessary. Complicating things is the fact that doing oil changes is one of the few do-it-yourself maintenance tasks that is still within the ability of the backyard mechanic to perform.

Recreational Oil Changing
The term "recreational oil changer" was coined to define people that change their oil far more than necessary because they actually enjoy doing it. It's easy to understand the psychology behind the recreational oil changing. It's the visceral feel of the tools, the victory when that old oil filter breaks free, the hot dirty oil pouring out, the joy of oiling of the gasket on the new filter, that new copper or fiber gasket on the drain plug, the clean clear oil going in, and the sense of accomplishment when you start the car, the oil light comes on for a moment, then goes out. For $8-10 in oil and parts, it's pretty cheap entertainment, but if people would be content to do it only when it provides some benefit to the vehicle it would be better.

The 3000 Mile Myth
The 3000 mile oil change interval has been pounded into people's heads for decades. It had a scientific basis when engines used non-multi-weight, non-detergent oil. It no longer has any scientific basis, but it is still being promoted by certain entities, most notably the oil change industry in the United States. This myth is also sometimes known as the "Cheap Insurance Myth."

The 3000 Mile Fact
There are still vehicles that need 3K oil changes, but it's not because the oil goes bad after 3K miles. One example is the Saturn S series. These vehicles have a timing chain system that is very sensitive to clean oil because oil pressure is used as hydraulic fluid to ratchet up the timing chain tensioner. If varnish forms in the timing chain tensioner bore then this system can fail and the chain will become loose and eventually break. Dealers have gone as far as tearing out the normal service schedule (6000 miles) and leaving only the severe service schedule. If your engine is destroyed (under warranty) by a failed timing chain then the dealer will legitimately request evidence of oil changes. Unfortunately this problem usually won't manifest itself during the warranty period.

The Dark Oil Myth
Dark oil does not indicate the need for an oil change. The way modern detergent motor oil works is that minute particles of soot are suspended in the oil. These minute particles pose no danger to your engine, but they cause the oil to darken. A non-detergent oil would stay clearer than a detergent oil because all the soot would be left on the internal engine parts and would create sludge. If you never changed your oil, eventually the oil would no longer be able to suspend any more particles in the oil and sludge would form. Fortunately, by following the manufacturer's recommended oil change interval, you are changing your oil long before the oil has become saturated. Remember, a good oil should get dirty as it does it's work cleaning out the engine. The dispersant should stop all the gunk from depositing in the oil pan.

The only real way to determine whether oil is truly in need of changing is to have an oil analysis performed. Since most people don't want to bother with this, it's acceptable to err heavily on the safe side and simply follow the manufacturer's recommended change interval for severe service. There are still a few cars that specify 3K intervals for severe service, but not many. If you look at countries other than the U.S., the oil recommended change interval is much higher than even the normal interval specified by vehicle manufacturers in the U.S.

Severe Service versus Normal Service
Each manufacturer specifies what constitutes normal and severe service. Generally, severe service consists of operating the vehicle in a very muddy or dusty areas (because dust particles get through the air filter and contaminate the oil more quickly), operating the vehicle in a very hot areas (heat breaks down oil more quickly), using the vehicle only for short trips in cold weather (the moisture in the oil never gets vaporized), or using the vehicle for towing or when carrying a car-top carrier. You'll often see claims such as "everyone falls into the severe service category," but these claims are untrue (follow the money and see who's making these claims). If you primarily do freeway driving in moderate weather you do not fall into the severe service category. If you're in doubt, the best way to see if you fall into the severe service category is to have an oil analysis done at the mileage of the severe service interval. Many people just like to play it safe and follow the severe service schedule, which is fine, but there is no benefit in changing the oil sooner than the severe service schedule states.

Different countries have different maintenance schedules, even for the same car. This fact has been the cause of long argument threads on Usenet. How could the exact same car need a different service schedule simply because of where the vehicle is used? At least part of the reason is due to the differences in fuel. For example, the U.S. and Canada has fuel with high sulphur levels which can cause more oil contamination. Japanese fuel has very low sulphur levels. Europe is in-between. Some of the newer engine technology (direct injection) which raises fuel economy, requires low sulphur fuel. Of course the oil companies have a vested interest in not lowering the sulphur as it adds to refining cost and enables more fuel efficient engines. Since "Big Oil" is in bed with the un-elected president in the U.S., don't expect any action of lower sulphur fuel for a while in the United States. If Al Gore is re-elected in 2004, and the Supremes don't simply ignore the election results again, then there is a chance for lower sulphur fuel in the U.S. beginning in 2005.

Oil change intervals with synthetic oil
Synthetic oils withstand higher temperatures before breaking down, and have more base stock and less viscosity modifiers. Synthetics wear out, become acidic, and eventually become saturated with suspended soot particles, just like regular oil. Again, an oil analysis is a good investment to determine the optimum oil change interval. Never exceed the manufacturer requirements for normal service.

Filter Change Interval
Back in the days of 3000 mile oil changes many manufacturers recommended filter changes only half as often because the filter did not become clogged with dirt at only 3000 miles. This was good advice back then, especially because with non-detergent motor oils a lot of Advantages of Synthetic
Synthetic oil was originally developed for high performance racing engines. Mobil tried to popularize synthetic oil for passenger vehicles back in the early 1970's. At the time, Mobil was promoting 20K or 25K oil changes with synthetic, but they soon backed down from this. Synthetic oil is a good choice if you have a vehicle with a high performance engine (in fact synthetic is required for many of these engines). It is also a good choice if your vehicle is operated in extremely cold climates. It has higher resistance to breakdown caused by heat and it flows better in extreme cold. Unfortunately for the synthetic oil industry there is virtually no advantage to using synthetic oil in a non-high performance engine that is operated in moderate climates. You probably could go a bit longer between oil changes with a synthetic, i.e. following the normal service schedule even if you fall into the severe service category, but I wouldn't advise this. In short, synthetic may give you the peace of mind of knowing that you are using an oil that is far better than necessary for your vehicle, but it won't reduce wear or extend the life of the engine. The mistake some people make it to wrongly extrapolate these benefits onto normal engines operated in mild climates, with the ultimate lack of any knowledge being manifested with statements such as "synthetics provide 'Peace of Mind,' or 'Cheap Insurance,'" or other such nonsense.

Extended Change Intervals
Most manufacturers of synthetic oil advise users to not exceed the manufacturer's recommended oil change interval. Part of this is self interest (they don't want to be liable for any damage) but the real reason is that synthetic oil, while it does have certain advantages, still becomes contaminated.

Be extremely wary of synthetic oil companies that offer to pay for your repairs if it is determined that their oil and their extended change interval recommendation caused the problem. Think for a moment of the incredible hassle you would have to go through to prove responsibility for an engine problem. Who would pay your legal bills? Who would pay for replacement transportation during the battle? The more bizarre the warranty the poorer the product is a good rule of thumb.

API Certification, Phosphorus & ZDDP
Never use a non-API certified synthetic oil (there are many of these on the market). The problem with the non-API certified synthetics is that they contain too much phosphorus (in the form of the additive ZDDP (Zinc Dialkyl Dithiophosphates)). The API has limited the amount of phosphorus because phosphorus shortens the life of the catalytic converter. These oils are fine for snowmobiles, motorcycles, and older cars that don't have a catalytic converter, and the extra ZDDP does provide additional wear protection. Unfortunately, the marketers of some the non-certified oils do not explicitly and honestly state the reason for the lack of API certification. You can check the status of API certification on the API web site. Be certain to go not just by the manufacturer name but by the actual product as well. This is because a manufacturer will sometimes have both certified and non-certified products. Suffice it to say that Mobil 1, Royal Purple, Castrol, & Havoline all make synthetic oils that are API certified and that can be purchased at auto parts stores and other retail outlets. Amsoil has one product line, XL-7500 that is API certified, but it's other lines contain too much ZDDP to be certified and should not be used in vehicles with catalytic converters.

the sludge remained stuck to internal engine parts rather than being carried in the oil to the filter. Nowadays the filter should be changed at every oil change. There are some people who believe so much in synthetic oil that they change filters without changing the oil. There's no harm in changing the filter without changing the oil, but there is no point in doing this.

 

Interesting article. But there's something a little off about the numbers.

Back when people used single weight non-detergent oil, the customary change was 1000 miles, not 3000. I remember old-timers shaking their heads when car manufaturers started to recommend 3000 miles. Much the way later old-timers shook their heads over the even longer intervals suggested for synthetics.

Detergent oils weren't immediately accepted. Tom McCahill used to rant against them, claiming that sludge build-up was a natural and good thing, much like the build up of a crust in a favorite pipe.

In the days of changing filters every other oil change, we did not have full flow filters. Part of the oil bypassed the filter. These days filters are full flow, so they do get more of a workout. That's got to be a factor in changing the filter every oil change.

3000 miles isn't that out of synch with what car makers recommend for severe service. Mazda recommends either 3750 or 5000 for the 8, depending on where you live. The factory severe service recommendations for my other two family cars (a Honda and a Toyota) are in the 3000 to 4000 range.

Ken