Motorcycle oil viscosity: What's optimal?

[Blake Sobiloff]

Here's a link to the SAE paper that lead to the JASO MA spec:

SAE paper

What's most interesting in this paper, to me, is not that moly can cause problems for starters and clutches. Instead, I'm thrilled to actually learn what viscosity oil the Japanese manufacturers use when designing and testing their motors!

I've been wondering for quite a while whether the manufacturers are aware of the rapid viscosity loss that almost all oils undergo in a shared sump environment, and whether they planned for that when recommending an oil viscosity and change interval. I mean, several manufacturers are claiming you can go 6,000 miles between oil changes, so surely they'd design their bikes to run just fine on 30-weight oil, right?

Well, not really, according to this paper. It's true that most manufacturers use a 10W-30 when designing and testing their engines, and I infer that their motors would run just fine on 30-weight oils. But, they've also seen significant pitting of the transmission gears when running 30-weight oils that are on the thinner side of the grade.

This is most likely why the manufacturers recommend 40-weight and 50-weight oils--not because of the engine, but because of the transmission. This makes sense if you look at the comparative viscosities between engine oils and gear oils. The most common gear oil weight is a 90-weight, which runs between about 13 cSt and 24 cSt at 100 C, while a 40-weight motor oil can be as thin as about 13 cSt and a 50-weight motor oil can be as thick as 22 cSt.

So, the most optimal viscosity oil for a motorcycle is likely to be the one that is as close to a 30-weight as possible while still protecting the transmission gears from pitting. This will vary from bike to bike, but you most likely do not want to let your oil's viscosity drop down into the 30-weight range, otherwise you risk damaging your transmission gears.

My take-away from all this is that I'm on the hunt for the most shear-stable 40-weight oil.

 

[Mackelroy]

I've been running Redline oil for the last 1000 miles in my bike(highly Moly fortified), and excellent clutch performance.

I've used many types of oils, but on an mc that turns any rpm is good for the most 2,000+ miles.

All you have to do is look at the MC analysis's of most , the metals in oils of those running higher the than 2 ,000 mile intervals and the metal content is pretty high.

IMO best to flush it out more frequently, I run the high end stuff cause you don't end up with oil shelacking buildup after many thousands of miles.

It's good to know the manufactures expect enough protection in the 30 weight region though.

 

[sunruh]

blake,
you also MUST consider ambient temps when choosing an oil. typically the bikes manual will show what weight of oil for what temp ranges. as an example, my bike manual shows using 40wt up to 105F. that means i cannot use a 40wt for at least 3 months of the year and must use a 50wt oil. but that is because of my ambient temps do to where i live. adjust the weight accordingly to your manuals recommendations in conjunction with your ambient temps.

 

[RiverRat]

This is why I can't use the 20W-50 or even 15W-50 during the fall/winter/spring in Idaho. It just gets too cold here and my bike just doesn't like the heavier oils. It makes a nasty clatter on startup and then takes a long time to warm. However, I have noticed none of this with the 10W oil. The manual reads 10W-40 so I use the M1 MX4T. If I lived down south then I'd use M1 20W-50 year round. I know the 10W's will shear faster so I'm going to be testing this MX4T to see how long I can safely run it before it will shear out of grade.

 

[Blake Sobiloff]

Sunruh, I agree that ambient temps are important as well. Where I live, the highest average high is 82 F and the lowest average low is 41 F. My owner's manual makes no different recommendation for oil viscosity based on temperature, so a 40-weight oil will probably work quite well for me. I suppose if I ever do a Baja trip in the summer I'll switch to a 50-weight for that trip, though.

 

[Jaybird]

My thought on the whole thing is, we need to be more concerned with the add pack of a two stroke (or shared sump) fluid, and not worry as much about keeping the fluid in grade.
I think we should have the fluid out of the bike before it goes out of grade.
Trying to extend an oil change for a two stroke is simply puttng the engine in jeopardy. Too much particulate, even if the fluid is still in grade.

There needs to be a slightly different train of thought on two-stroke changes, as opposed to other applications like autos.

 

[sunruh]

jay,
while i agree with what you are saying (mostly), it is not possible to achive. some guys would litterly have to change oil in mid-race to prevent out of grade oil from being in their motors.

as you know, 2smokes only have to worry about the tranny and typcially an oil like sae30 or sae40 (or atf type f) is perfect for this. however, 4bangers with shared oil (crank and tranny) have a much bigger and broader range to consider. as one of my uoa's have shown, shared sump oil can go out of grade in as few as 40 miles. nothing quite like changing the oil before needing to refill the gas!

 

[Jaybird]

I agree...
Exactly why I place so much importance on the add pack. Even when the oil is in grade, the high heat/rpm's, and components like the solid lifters and plain cam bearings of a racing 4 stroke, make it completely neccesary that the oil have a very robust boundary make-up.

Your analysis reports have really opened my eyes. I would not have thought that some fo the oils you used would have gone south as fast as they did.
I think anyone who is interested in performance engines of any flavor, should really take notice of racing motorcycle reports.
They have to be the oil chewing leaders.

 

[Titan]

Blake, nice paper. My take is that the Valvoline 10-40 Motorcyle oil is maybe the best. It goes out of grade more quickly, so, change it frequently...since it's relatively cheap, no big deal. Otherwise, it performs admirably in the other areas for a watercooled bike.

 

[Titan]

Actually, I just ran across some Older Delo 400 without the C4i+ designation, so, I bought the 7 quarts that they had of just C4i designation. Hopefully, this will be good for my wet clutch, as I understand some people think the newest Delo 400 with the C4i+ is a bit too slippery for certain wetclutch applications. I'll know in a few weeks when I change to this 15-40 from Valvoline 10-40 Motorcycle oil.

 

[Gary in Sandy Eggo]

Blake Sobiloff Writes: My take-away from all this is that I'm on the hunt for the most shear-stable 40-weight oil.

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I ride an air/oil cooled Intruder 1400. I DO use the most shear-stable 40-weight oil. It's Pennzoil automotive SAE-40. (I may switch to Haviland because of the higher flash point - closer to 500F as opposed to Pennzoil's already high 450F.) With the exception of VII polymers, the additive package is the same as multi-weight oils. No loss of viscosity in motorcycle engines here.

My change interval is 5000 miles. Oil consumption in that period of time is less than half a cup of oil out of the 5 quarts that the crankcase holds.

Regards, Gary in Sandy Eggo