Nathan Kwok

FWIW the manual recommends 10w30 but NOT 10w40. It seems every engine builder recommends 20w50. Heavier weight oil provides more protection under extreme driving conditions, and it will also help symptoms of loose tolerances in the engine. So if you track your car or your engine is a little tired, those are good reasons for heavier weight. Mazda based their 10w30 recommendation under "normal" driving styles, ie driving on the freeway and around town, the stuff most people do. Lighter weight oil will flow better and decrease initial wear when cold and provides better gas mileage. Keep in mind that for normal cars, the majority of wear occurs during startup, but for cars that see a lot of tracktime, its different.

Fangman

Not to start an ego war or anything, but different weights of oil do not absorb different amounts of air, or water for that matter. And, since they are liquids they are essentially non-compressible, so compression isn't necessary to move them around. The numbers used to identify different oils are based on their viscosity characteristics. There are standard curves of viscosity vs. temperature for each of the "straight weight" oils (SAE 30W, 90W gear oil, etc.). Each of those oils performs (visc. vs. temp.) as the standard curves show. The multi-grade oils aren't really any different, except that the slope of their viscosity-temperature curve is different than the standard, so that at low temps (I don't recall exactly what the definition of low temp is) the oil has the viscosity of the lighter grade (first grade weight), but at higher temps it acts more like the heavier grade (second grade weight). You want relatively thin oil when the engine is cold to ensure that it flows, but also to reduce viscous drag in all of the bearing surfaces. At higher temps, you want a thicker oil to improve the lubricating film strength (i.e. ensure that the oil film remains between the bearing surfaces - if not you end up with metal-to-metal contact which is obviously not good.

BTW, I'm a chemical engineer with a number of years of refinery experience. The basics of the above discussion are correct.

RonKMiller

Fangman - Nice to see someone with CREDENTIALS answer this question. I live in Tucson and run straight 30W dino juice year 'round. Why, I dunno......... it just seems to be the right thing to do - run an oil without uneeded additives especially when your motor lives in a warm to hot climate all the time. What are your thoughts on this?

ttb

so 10w40 is really bad for the FDs? i got my about a month ago and there's a little service oil change reminder that has 10w40 on it

juliof

Just to remind you synthetic users there are many brands that are claiming there oil is synthetic when its not, one brand for sure is Castrol Syntec and semi synthetic Castrol so beware.I read Mobil1 and Exxon are still using real synthetic oil base stock , I dont know about the other commom brands Penzoil,Quaker state and Valvaline.

fdracer

I run 10W30 all the time. I think 20W50 is perfectly fine for the motor, but have you guys considered the turbos. That is an awfully thick oil for the turbos to rotate in, and that is specifically why I run 10W30. Just wondering, but how many of you guys have had your turbos go south on you using 20W50? I'm not an expert, but on the piston turbo cars I run, I have found 0W20 synthetic works best, the turbos pull like there's no tomorrow. I'm even considering using 0W5 Neo synthetic race oil. Needless to say these cars don't have the heat problems of a rotary, but I think the turbos should also get a lot of consideration when choosing an oil, considering their spinning at a much greater rpm than your engine.