Thin vs thick

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Why do so many use 10w40 to 20w50 and thicker oils in motorcycles?

I understand todays cars run 0-20 and are recommended.
Aren't motorcycle engines mainly built a little better than cars, in order to turn the rpm's they do?

Is it the shearing action from the gearbox that requires thicker oils to keep from going out of grade?

With a shear oil, could you not run a 30 or even 20 weight?

Just wondering as since I joined this site everything I thought I knew about oil went out the window!
 
Read the heading of this post and had to come to see what this was about.

Thank god it was about oil.
 
I initially ran 10W-40 OEM oil in my Kawasaki Brute Force (it came with 10W-40 from the factory) and it sheared down some. The BF shares engine oil with the transmission (but no clutches). While I cannot speak for others, I did not care for the shearing, so I am trying 5W-40 T6 in it to see if this oil will stay in grade.

If not, then I have a new mission
crackmeup2.gif
 
Originally Posted By: Sonataman
Why do so many use 10w40 to 20w50 and thicker oils in motorcycles?

I understand todays cars run 0-20 and are recommended.
Aren't motorcycle engines mainly built a little better than cars, in order to turn the rpm's they do?

Is it the shearing action from the gearbox that requires thicker oils to keep from going out of grade?

With a shear oil, could you not run a 30 or even 20 weight?

Just wondering as since I joined this site everything I thought I knew about oil went out the window!



Thicker oils are not any more or less prone to shear out of grade.
 
Air cooling might have something to do with it. The latest injected and water cooled engines tend to specify thinner oil.
 
My 2012 Honda TRX400X specifies 10w30 year round with 10w40 being an alternative in 100+ degree heat. Honda even allows a 5w30 in the cold. Just a few years ago, Honda recommended 10w40 year round and allowed up to a 20w50 in this motor. It is 397CC, air cooled, four valve and spins up to 9K. Total sump capacity is 2.2 quarts. I can turn Honda OEM 10w30 into water in about 2.5 hours and Valvoline ATV 10w40 is junk after maybe 3 hours. I ran SRT 15w40 and noticed much improved shifting.

As an experiment, I ran Rotella T5 10w30 (synthetic blend) this winter. I had roughly 10 hours on it when I dumped it. It literally drained out like water, no joke. While I never noticed a degradation of shift quality, I am not comfortable with my oil draining out like water. It was still "oily" feeling, but nothing like SRT 15w40. I will be running SRT 15w40 or ST 15w40 from now on, all year.
 
Clearly, air cooled engines can, at times, run hotter. I'd bet that a "hot" air cooled engine's "thick" oil is no "thicker" than a water cooled engine's oil.

Aviation engines, for example are air cooled and certainly lose oil pressure when oil temps exceed 240 degrees. The mandatory 50 viscosity oil is also able to protect a flat tappet camshaft without extreme pressure additives.
 
I never look or care that my oil drains like water. Of course hot oil when drained will act very thin. I run Rotella T5 10W30 as 10W30 is recommended by Honda and had great wear numbers and the oil was still in the 10w30 range, although the lower end, even after 5000 miles. There's the old thought thicker is better that lingers on from decades ago and guys run thicker oil and it works fine, thinner works fine too. It seems like oil generally works fine anymore in any weight.
 
My 1986 BMW K75C (No tranny gears in crankase, dry clutch, watercooled) tends to shear it's 20W-50 oil quite a bit. I ran 5W-40 Mobil 1 TDT and it was a THIN 30Wt after 6000miles.

This bike does not HEAT the oil too bad however it does shear everything I put in there a grade or more. Not sure I want to run any 30wt oil in this bike.
 
My 2006 ST1300 manual lists 10w40 and 20w50 as recommended oils. The later models of the bike list 10w30 as recommended. There have been no changes to the engine to cause the recommendation to change, other than a steady migration to thinner oils. It has been noted that 40 weights shear to a 30 weight, but that 30 weights tend to stay in grade in the ST. While that's not a blanket statement across brands of oils, or brands of bikes, my experience does not mirror dparm above. I've seen several brands of "thicker" oils (20w50) that shear out of grade both faster, and more severely than 10w40's in motorcycle's with shared sumps. I think manufacturers try to cover their behinds by recommending thicker oils knowing they will shear over the recommended OCI.
 
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There is much to oil viscosity determination. One consideration is that the bearing surface has sufficient surface area to carry the load with the oil film area available. A larger bearing with the same load will have a lower psi so less load on the oil film that keeps the parts separated. Other things remaining equal, higher viscosity oil has higher film strength. Also a consideration is the actual hot oil viscosity at the hottest points inside the engine, not the average oil temperature in the sump.

I'll stick with the engine designer's oil viscosity recommendations until someone else does the experimentation.
 
I don't know why some riders are spring loaded to 40wt or
50wt... the difference between the parasitic drag between a
free flowing 30wt versus the thicker 40wt 50wt can be noted
as much as a 3HP gain on a dyno and thats the difference
between a 30wt and a 40wt... imagine the difference between a
30Wt and 50wt... mercy!!!

Quote SportRider...


Some oil manufacturers and their representatives claim that using
their product will result in more horsepower. These are special
ultra-lightweight-viscosity racing synthetic oils that are said to
reduce the parasitic drag that oil has on an engine's internal
reciprocating components. We decided to put these claims to the
test-an actual dynamometer test. Two of the full synthetic oils in
this test make these horsepower claims on their labels: Maxima Maxum
Ultra (in 0W-30 and 5W-30) and Motul Factory Line 300V (in 5W-30). We
took two open-class sportbikes-a Suzuki GSX-R1000 and a Yamaha
YZF-R1-and ran them with common off-the-shelf Valvoline 10W-40
automobile mineral oil to set a baseline dyno run. That oil was
drained and replaced with the 0W-30 Maxum Ultra in the Suzuki, and the
5W-30 Motul 300V in the Yamaha. After about 15 miles of running to get
the oil fully circulated through the engine, the bikes were then
dynoed again.

Lo and behold, both the Suzuki and Yamaha posted horsepower gains.
While not an earth-shattering boost in power, the gains were far
beyond common run variations, and weren't restricted to the very top
end. The GSX-R1000 posted an increase of 3.3 horsepower on top, with
some noticeable midrange gains as well; even more interesting was that
the power steadily increased for several dyno runs (as the coolant
temp increased). The Yamaha responded nearly as well, with a 2.7
horsepower boost on top.
It should also be noted that while riding
both bikes, there was a noticeable ease in shifting with the synthetic
oils compared to the automobile mineral oil. Pretty impressive for
just changing oil, in our opinion.
 
I was curious myself & what I have researched here and a few H-D & Buell forums is that air cooled engines like the one in my Buell Blast (20W-50) and my scooter (15W-40) can become hot during longer runs and idling in warm/hot weather so they recommend oils that are thicker to provide better protection and resistence to shearing; also motorcycle oils, racing oils, and heavy duty 15w-40 oils can contain more antiwear additives than passenger vehicle engine oils (higher amounts of ZDDP have phosphorous that can damage catalytic convertors) which help protect internal engine parts/surfaces of air cooled motorcycle/scooter engines
 
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