rotarygod

It's not that simple.

bse50

That's exactly why i'm asking!

rotarygod

This will really confuse you! I am going to compare the above Amsoil 20W50 to Valvoline Conventional 20W50. To keep things simple I am going to use the same 3 temperatures although my chart goes from -4*F up to +338*F. When I see someone say that a specific weight oil on a bottle is "too light" or "too heavy", I laugh! How do you know? They aren't all equal! If I compare 5 other oils of the same weight I am going to get 5 different sets of numbers! Some of these numbers will be closer to oils of different weights from other manufacturers as specified on the bottle. The sad thing is that now that I posted this, someone is going to look at these 2 oils and try to draw a conclusion from it since one is conventional and one is synthetic. Don't do that! At least not entirely. You can gather some good data from it though. We can tell that at 212*F each oil is essentially the same viscosity. They should be. The second number, 50 in this case, is based on cSt at 100*C that they are shooting for. It doesn't mean that this is what temperature you oil should always be used at though. The first number doesn't have to mean it goes down to an equivalent 20W oil and it doesn't. Not even close. What we can see from these numbers is that the synthetic will definitely flow better at lower temperatures which means less drag on the system. This means less power loss and better fuel economy during startup. Is it any wonder automakers are slowly recommending synthetics more and more? A higher number down here doesn't necessarily mean better protection though. They each have PLENTY of viscosity to them.

These numbers are very different because the viscosity index is different between them. Havoline 10W40 has a VI of 151. Their 20W50 has a VI of 121. The Amsoil 20W50 has a VI of 172. A thicker oil as labelled on a bottle doesn't necessarily have a higher or lower VI as weight changes. Sometimes the next higher oil will have a higher VI and the one above that is lower again. It is complicated but just going by numbers on a bottle to draw conclusions doesn't work.

Amsoil Synthetic 20W50
50*F : 576.85 cSt
212*F: 18.84 cSt
248*F: 8.74 cSt

Valvoline Conventional 20W50
50*F : 1168.35 cSt
212*F : 18.43 cSt
248*F : 11.51 cSt

bse50

I think I can understand this data and in the end they have pretty similar viscosities at 212°F.
I tend to like royal purple's 10w40 oil for my climate and use (mostly track use) because my retarded mind thinks that an higher viscosity will help reduce wear at higher rpms.

Going back to the 2 amsoil blends you mentioned could you please tell me how the difference (in percentage) from 6.2 to 8.74 relates to engine lubrication and bearing protection?
I think that it is a fair point to discuss since that's usually the reason why an oil is chosen.

rotarygod

Keep in mind that difference was at a very high temperature more indicitive of the insides of the rotors and not the bearings. The bearings would be cooler and hence the viscosity there would be a greater distance apart. What I'm getting at by showing you those numbers though was that at that temperature, 2 oils of the exact same bottle weight had a greater viscosity difference than the 20W50 Amsoil and the 0W20 Amsoil did. At 212*F of course the 0W20 is far different. I also mentioned that an oil is considered useful down to 1.77 cSt. That part is basically best described by saying "it depends". Bearing lubrication can't be determined by just analysing viscosity at that point as the viscosity isn't only what is important but rather the formulation of the oil. It plays a role but don't base your assessment only on viscosity. As I said, it's complicated.

We need to look at shear strength of an oil as well. Viscosity isn't the only thing protecting the bearings. Actually viscosity is really only protecting anything below about 160*F as above that point the additives in the motor oil start to become active and do their jobs. Incidentally for every 15*F you go above 160*F oil temperature, your oil life cuts in half. 175*F has half the oil life as 160*F. 190*F has half the oil life of 175*F oil and so forth and so on. Having a higher oil temperature means it needs to be changed more often. This alone should tell you that different people should probably exercise different oil change intervals than others.

Here's something else to mess with some people. A higher revving engine at full throttle needs a THINNER oil than a street car that gets lugged around at low rpms all the time. If you track your car on weekends and drive it harder then, you need to change to a thinner, not thicker oil! Here's a bit of information that will seem counter to this though. The hotter your average oil temps, the thicker the oil should be! That's an issue of oil cooling at that point.

A rotary really complicates things. Our oil is responsible for all of the rotor cooling, it lubricates apex seals and has to burn in the combustion chamber as a result, it has to lubricate bearings, and it has to lubricate stationary gears. That's a lot of different jobs and different types of stresses that it is subjected to. On top of that we have a wide rpm range to deal with. It these engines were all stationary generators, recommending a single oil type would be as simple as asking what temperature will the oil always stay at. However in a car this is always changing which means we need to have a dynamic oil with good additive properties. I personally like synthetics the best as they can cope with this dynamic far better but that's just me.

The more and more you get into the technical aspects of oil, the more you'll see why driving style, oil cooling, and climate all play a role in what oil should be used. It is not good enough to say all rotaries should use a 20W50 oil. Besides as I have shown you above, not all 20W50 oils are equal to others. Some may have properties of another brands 10W40 or overlap in a far greater way.

The more info that gets out there, the more questions are raised.

bse50

Point taken, thanks
The reason why I use a 10w40 oil is because of the temperatures my oil works at. If I were seeing lower temperatures i would probably take a lower viscosity oil to get better higher rpms lubrication as you say.
Guess that it is a kind of compromise as anything else in the automotive world and that sucks.
Thanks again!

rotarygod

As a general statement, a car that has lesser oil cooling should run a thicker oil. RX-8's equipped with 1 oil cooler should run a thicker oil than those equipped with 2. Those in hotter drier climates such as Arizona or parts of Australia should run a thicker oil. This is one reason why the Arizona automatics had so many engine failures a few years back as they were running on 5W20 and only had 1 oil cooler. The fact that the oil injectors in the series 1's don't lubricate the center of the apex seals very well is what sealed the deal for those engines. A good synthetic oil could have helped but ultimately wouldn't have prevented their issues. No oil is good if it can't get to where it needs to go!

PhillipM

And to make that even more fun, thinner oils generally cool better than thicker ones...

Nubo

Saints and ministers of grace, defend us!

jl1rx7

iTrader: (2)

Rotarygod, your right for every answer you make with oil a new one is raised. It seems like if we could seperate all the different functions that oil has to do on these engines could you get the best oil for that job. Otherwise like as others have said all you are doing is tradeoff's. Thanks for all the info! I still think I will stick to what has worked for me over the years with my 7's. Hot weather 20w50 castrol. Cold 10w40. With the 8 from what I have read you can't go wrong if you use 10w30 for moderate weather. 20w50 for hot climates.

jasonrxeight

meh, beer and wine, at the end of the day youre just as drunk.

nycgps

iTrader: (1)

even with 2 oil cooler and 40 degrees out, I constantly see oil temp going around 95-105 Celsius. if its 5w20. hohoho ~

I can't imagine what 5w20 looks like if Im in Arizona ... ooh right, Mazda said 5w20 works so it must work ?

twistedwankel

iTrader: (6)

The only difference between synthetic and dino multi grade is the number of cylces it can go from 5-20 or 10-40, etc. And the way it burns in your combustion chamber. You like residue? You have rings or seals?

Some Synthetics are not equipped with decent pH modifiers either which is not good for the parts as acid builds.

I don't care where you live your engine will be the temp that your cooling system and oil keep it...DOH.

Replace your crappy radiator and antifreeze. Change your oil frequently if you drive in a dusty town.

I have no reason to use other than SL rated dino changed no more or less than every 6 months. I will increase the oil wt on top end (5W-30) if I start burning too much oil.

The 5W-20 is chosen for fuel economy for which the Wankel is not well loved.

Modern oils and additives are totally superior to yesteryear. They can withstand much higher stresses and temps.

Don't mix brands. Mix wts if you want. Add one quart of Synthetic to 4 of dino if you want (like the oil companies do in blends).

I run the crap out of my cars but always check the oil level before I do it. If your dipstick is in your hand you are not driving her home.

HiFlite999

iTrader: (2)

As I've said in other threads, going to a "heavier" oil with the stock Series I oil system will cause the oil pressure bypass to open sooner and thus reduce oil flow compared to a "lighter" oil in that range of operation. Granted that the viscosity labels are misleading, but I'd think more than twice about 20w-50 in a stock series I.

For the most part, even up to 85 deg F weather, I seldom see more than 165 deg F oil temps, measured at the base of the oil filter with a reasonbly well calibrated gauge. As Rotarygod hints, these low temps may be a problem, and perhaps (for average-climate dwellers) a more common one than running too hot. (ref: milky oil problem).

RG: What's you magic chart have to say about Castrol GTX 5w-20 vs. 10w-30?

Texas RX8 R3

haha!

bse50

You should stop posting this kind of misinformation. Synthetics have evolved during the years. Actually a synthetic often burns cleaner than a mineral oil nowadays... go figure. When you tear an engine down you can notice where the oil injectors dip the oil on the rotor's face because i'ts cleaner than the rest of the rotor. What leaves 99.9% of the residues is fuel.
On top of the shear stability a synthetic also offers better bearing protection because of its formulation.
What you should state, in case, is that since not all synthetics are created equal you should pay attention to HOW a determined brand mixes with fuel. That's all that matters from a seal lubrication point of view.
Mobil1 mixes poorly and lubricates poorly, other brands such as amsoil, royal purple, redline, valvoline, some castrols etc mix pretty well, burn clean and work perfectly as they should. That's the only reason why mazda recommends the use of dino\blends. They can't say "don't use this and that brand".

Also keep in mind that we're not talking about some old 10a\12a engines, swelling of the seals and corrosion has not been a real problem ever since!

twistedwankel

iTrader: (6)

Misinformation? These cars are now running 10:1 compression. Way more than my piston cars and the old 13B. ALL lubricants have progressed over the years. ALL I said is that I personally am going to follow Mazda's written recommendations but not "blindly". Everyone has different driving styles/conditions. My car will NEVER see a temp lower than 40F or higher than 95F. I am a fan of valvoline or pennzoil simply because I have never had a problem with their better dino oils. EVERY study I have ever read says there is no evidence that there is any significant benefit to using Full Synthetic vs Quality Dino changed at proper intervals. That said I use Mobil 1 15W-50 in my '81 air/oil cooled perfomance bike and even the antique clutches love it at 9k shifts. Recommended by a friend who is a certified mechanic for bikes.

Obviously the FL PO of my car did things wrong as he got a new engine with ONLY 55k on the car. He was using various synthetic most of the time so can't blame any one brand. So I now have a brand new motor under warrantee to pound extensively on the autocross courses and maybe a couple track days.

In 48 years of driving I've never had an engine fail and I've raced 'used' cars for the last 30 years. The very first thing I did after replacing the crappy brittle OEM plastic radiator was change the oil and filter.

Bottom line is that it's hard to justify the high cost of quality full synthetic unless they are your "sponsor".

I state my "opinion" and you state yours. It's called a "discussion". One guy says he's going to use the cheapest 10W-40 dino he can find. I have no issue with his comment. I found it amusing.

Number one RX-8 question I get while racing? "Hey you got any extra oil? I forgot to bring some."

rotarygod

If you haven't seen a study that shows an advantage of synthetics to conventionals at regular oil change intervals (what is regular anyways?) then you haven't seen a reputable study!

Most mechanics are idiots. Keep that in mind when you listen to one. Most of them don't know how and why. They just know how to unbolt it and bolt another back on (usually). I got in an argument with a very well respected mechanic who had been doing it his whole life. He was tryling to convince me that independent suspension has no advantage to a solid rear axle. That's the genious of the average mechanic!

nycgps

iTrader: (1)

you can't be serious.

nycgps

iTrader: (1)

there is a significant difference between "discussion/opinion" and "bullshit"

anybody can call their opinion "its just a discussion", but is it really? that's another thing to consider.

bse50

I'm sure he is. I bet that this mechanic's mustang covered the quarter mile in 3.5 seconds so independent suspensions must be useless!