Someone at the shop asked what type of weight he needed for his old bmw. As a joke I said in the background," If your engine is really old, you might want to go heavier, I run 20w-50 in my rotary!"

Why was that? My friends rx8 they recomend 20 weight form what I remember. Are the clearances that far apart that 20w50 is best?

 

cuz the mazda handbook and service manual recommends it?

 

Because you've got a rotating assembly sandwiched together on a long axis and secured by 24 inch long bolts. wobble wobble wobble.

 

Straight 20 is fairly thin, I'm thinking you are remembering something incorrect from your friend.

I'm not sure about the tolerances, but rotaries run quite a bit hotter than piston engines. Heat thins oil, making it less effective, thus I think the thicker oil is specified due to the heat.

 

the Rx8 does run 5w-20. in my experience for a street car you just need something wet and slippery in there, and it doesn't matter if its 5-20 or 20-50, they both work fine.

in a race car you need at least a 40 weight.

 

Huh, I guess I'm wrong. Good to know

 

Although lower weights are known to have lower oil pressures...

 

Imagine your oil as maple syrup. Rotary engines run hotter than most cars, and 20/50 is thicker, therefore it'll lubricate better . If you live anywhere where it gets below freezing, use 10/30 in the winter because 20/50 will get so thick at those low temps that it can't do it's job at start-up.

 

starting in 2011 honda started using 0w20 in almost all of there motors. they claim better fuel effeciency and tighter engine clerences. on older cars i always go thicker. 20w50 in my rotary

 

RX-8's aren't FC's, I'm willing to bet they have tighter clearances. I use 10w30 in my car year round, I'm not too worried about getting caught in ambient temperatures above 90*F and below -10*F. (but in Missouri, anything's possible I guess.)

As for engine clearances vs. oil weight, I'm curious as to what the advantages would be if I switched to 20w50. My engine (like most rotaries I presume) has terrible blow-by. I run premix with a deleted OMP, and I usually have a quart or so more oil in the sump when I go to change my oil. Is this due to me running a lower viscosity? The same thing used to happen to the old mazda racing teams when they raced the Cosmo. I'd like to think mazda tightened the tolerances on engine parts since then though... Although that wouldn't matter in my case if I improperly built my engine. I tried to keep things tight...

 

there are a couple of things that cause the "more oil" thing, the first being a removed/non working PCV system.

the second is an engine with worn seals/housings.

so the oil really doesn't have a lot to do with it. there are a couple of rotary engine oil study SAE papers, and basically in a street environment, you can run anything, even ATF.

in a race car though, it needs to be about 40 weight hot, if you run thinner oil racing mileage gets better, but the bearings wear

 

Idk what was in my engine previously, but I just did an oil change to Castrol GTX mineral 10W-40 and man what a difference. The whole engine operation is just simply, smoother. Both cold and hot. Although, I baby it until at operating temps, keeping it below 2.5k when possible.