When is engine oil too thick?

5w20 to 5w30 is recommened for the RX-8 in various locations around the world.

I hear and read of people using 10w40 and 20w50. Under racing conditions or high revs it would seem there might be some benefit to using thick oil. But if it was too thick would the OMP have problem distributing it, would it "insulate" too much and cause the engine to run too hot, and burn up too much gas? What is the point of thick oil if its not doing the job of lubricating the rotors?

Is 5W30 really the thickest oil that should be use on the RX-8?

 

For normal street use, yes.

Keep in mind that the first number is viscosity when cold, and a good part of engine wear happens when a cold engine is started, so unless you live in the tropics you should not go higher than 5w whatever.

There has always been a concept that you should run one grade thicker than the manufacturer recommends. IMHO, that concept has been obsolete for decades. Probably ever since thin wall bearings became standard. If I was ever worried about film strength, I'd go to a synthetic oil of the standard weight. For a US model 8 in a temperate climate, I'd stick to 5w20.

Get ready for lots more opinions.

Ken

 

I'm wondering if any experts know if the RX-8 OMP can handle distributing much thicker oil.

 

its probably fine.

in Japan they have lots of *Rotary oil* that has higher than 30 viscosity, I think all of them are Full synthetics.

they didnt say for which rotary, they just list it as *RE* oil.

 

Since you understand japanese....

http://www.youtube.com/watch?v=lYEpUWm-S0Q

 

thats just like the video that we got. but in japanese.

http://www.hks-power.co.jp/products/oil/oil_index.html

and your point ?

 

I dynoed at 203 rwhp using 10w40 oil, on a mustang dyno

 

8 days ago I completed a 2190 mile road trip in 3 days,at an average running speed of 75 mph.My MOP pumped 0.75 quarts into the motor.I have 10W40 Castrol GTX in the motor(and have since day1).

 

Castrol makes a 10W60 that BMW uses in its M cars. Im thinking of trying it on a track day. I live by the stuff in my turbo miata.

 

wow, isnt 60 kinda too thick ?

 

I wouldnt think so if you are running on a hot track day. Id be more worried about 10 being too thick on a cold start. The oil is called tws motorsport, i havent seen it at any stores but you can get it at at bmw dealer.

 

In most markets, Mazda specifies 5W30. In the North American market it's 5W20. They've also replaced lots of engines in North America.

 

As you thicken an oil, its pumped volume goes down and, therefore, its cooling capacity goes down s does its injected volume.
Since viscosity doesn't have much to do with film strength any more, I'd be worried about using anything much thicker than 30 in normal places and 40 in hot places.
I'm going to screw around with some blending of weights on this next fill, so I'll see if there are any directly observable advantages to higher viscosity (beyond 5w-30).

 

The volume of oil delivered from the MOP is adjusted by changing the stroke of a plunger,I don't think the viscosity differences in the range being discussed will change the volume discharged at normal operating temperatures.
I know when I tested my Turbo II MOP delivery,the volume did not change using 10W30 to 20W50.Of course the plunger stroke control was considerably cruder than today's MOP and volumes of oil injected were greater.

 

The MOP doesn't have a volume feedback, so at equivalent pressure, less oil is injected at higher viscosity.
Effectively, viscosity is inversely proportional to density in a pressurized system.

 

The volume of oil from the MOP is determined by the diameter and the stroke of the plunger----think of a syringe!.The diameter is fixed and the stroke is adjusted by the PCM.The difference in volume pumped due to viscosity of oil(in the ranges being discussed cannot be measured by the normal tools available.Try it!!

 

OK. Now, think of a syringe (with a fixed stroke) and try it with alcohol and then with syrup.

 

i wish the thicker oil = better protection myth would really get killed one day

 

Well, there is a "magic" viscosity for any motor, which may or may not be borne out in the recommended value.
Certainly I have found that 5w-30 produces higher vacuum and compression numbers than 5w-20 across the board.
What that translates into with regards to wear is unknown.

 

Look if we were comparing alcohol and sugar I would have kept quiet,clearly that is not the proper comparison.
Anyway I will make you a deal----you have all the stuff needed to measure the volume of 5W30 through an oil injector.Send me your paypal address and I will send the money for you to buy a gallon of Castrol GTX 10W40 and you can test it for yourself---I defy you measure any discernible difference in the volume of oil passing through the injector.
As I said before I did thid test 10 years ago with my TurboII---the only difference being that the volume is controlled mechanically by throttle position.There was no difference

 

Now I use mobil-1 0w-30, but I"m thinking about changing to M-1 0w-40. One of the reasons is the hot weather, and the other is that the Mobil-1 0w-30 is "only" high performance oil, but the 0w-40 is the ULTIMATE in every sentence (protection, cleaning, stability).
But I worry about the MOP. How about oil nozzles with the thicker oil? Will those "carburate" the thicker oil properly? I don't believe that the RX-8 MOP can build up pressure.(after reassembled an RX-8 MOP) Only the vacuum (and a little part of the original engine oil pressure) let the oil gets to the chambers across the nozzles. So, maybe the injected oil amount will be less with the thicker oil. I don't know. ???

 

I'm thinking 10w40 DINO would shear down in really hot weather so that its more like 10w30 after a while.

10W40 synthetic though seems like it would maintain its viscosity and not get too much thinner or take a really long time to get thinner. Therefore the engine might not be getting its correct level of lube.

I do something weird little like MazdaManic.

I blend 5w30 and 5w20 Castrol synthetic together. I'm thinking it would be something like 5w25. My theory is that it would be a little thicker for protection, but a little thinner for better gas mileage.

If I'm out to really rev the engine than I would just go straight 0w30 or mix it with 10w40, so it would be like 5w35.

I also could have sworn somebody telling me about one of the famous rotary tuners in Japan selling 5W35 oil, so maybe its not so crazy.

I'm a bit concerned with going 10W40 synthetic. I could possibly see going that high with dino and in really hot weather though. But, in really hot weather, I think it would be better to use synthetic.