fyrstormer

Isn't that only true for dino oil? Synthetic oil molecules are much more complex. Also, I don't know what the nominal operating temperature of this engine is, but in my Passat it's 190F, which is well above what you say the oil can withstand for long periods of time. (and then there's the turbo, which glows orange after a good drive, so...)

ken-x8

Except for being cold and dry, air at 30,000 feet is the same as at sea level. Percent of oxygen is the same.

Ken

fyrstormer

Fair point. I was talking about the absolute quantity per cubic meter. I guess that doesn't matter as long as there is sufficient compression in the intake stage, though. In any event, the coldness and dryness are significant enough on their own to affect combustion byproducts, partly because with such a drastic change in temperature, the reactivity of the component gases are affected. Same concept as how cold, dense air will require more fuel, which will burn hotter and produce more NOx than warm, thin air does. (I think that's the way it works, anyway. Stupid brain and its imperfect memory.)

rotarygod

I never said that an oil can't withstand temps above 190*F for long periods of time. I did say that oil life cuts in half for every 15*F that oil temperature is above 160*F. Big difference. Basically the only definitive thing I have told you is that oils break down faster and faster as they get hotter and hotter.

This applies to oils as the concern is with the degredation of the additive package. The additive package is what makes a motor oil, a motor oil and comprises nearly 25% of the total amount. Some less, some more. This can and will break down on you meaning that you will be reliant solely on the strength of the base oil when it does. The additives really don't become functionally active until the oil reaches about 160*F in temperature and this is why the oil life statement starts at that point.

A synthetic base stock is superior to a conventional. A synthetic oil will maintain a higher level of protection while colder or during warmup and a higher level of protection as the additive package starts to breatk down. It slowly begins this the moment it gets hot for the first time. It just takes thousands of miles, or consistently hotter temperatures to do. At this point you rely on the base stock.

Amsoil's old claim about a 25,000 mile oil change interval had everything to do with the strength of the base stock as they claimed it was good for that long before breaking down. It probably was but at the severe expense of no active additives and being diluted by fuel and other contaminants. This is probably what ultimately led them to retract the 25,000 mile advertising and market some of their oils as being good up to 10,000 miles. This is more believable when it comes to the average driver on the street with a good formulation oil. Keep in mind this is primarily geared towards the synthetics as at this point you are somewhat relying on the additive package and somewhat on the base stock itself, not that you never are anyways.

If engine A runs with an average oil temperature of 190*F and engine B runs with an average oil temperature of 205*F, engine B will need it's oil changed in half the time. This is just an example and doesn't mean that it's oil is broken down in only a few miles or that it didn't even go 5000 miles in that time. It just means it's oil broke down faster.

HiFlite999

iTrader: (2)

IMHO, most oil changes are needed because of contamination rather than breakdown. But the 3,000 mile oil change is a waste of money rather than really needed. I've been doing the Toyota at 5-7k miles with no ill effects after 154k so far. The 8 I do at 3-4k though, because at least 2 quarts don't drain out, and have to wait for another 3-4k for their next chance to do so.

rotarygod

My Toyota daily driver has 10K oil change intervals. Full synthetic 0W20.

usnidc

I guess that is one advantage to having the rotary, we get to add fresh oil every 1000 miles or so...

fyrstormer

That's the advantage of my Passat's engine too. Turbos leak. I always have to top it off from time to time.