quote:
Originally posted by Jay:
arkainzeye, I think the reason motorcycle manufacturers specify 40 and 50-weight oils is because they fully expect them to shear down a full grade or more. We've seen that happen in several UOAs and seen it happen fairly quickly.
That doens't make much sense, heck depending on the base stock and the add package it could also thicken up one grade. Then if your running Xw50 you now have a 60wt.
I would say you want to use the thinnest oil, without losing any hydrodynamic lubrication where you have to rely on your barrier additives.
In the case of 20w50 vs 10w40. Most water/oil cooled bikes can use the 10w40 with great results. Most air cooled bikes or V-twins should run 20w50. The reason for the V-twins is becuase the rear cylinder usually runs hotter than the front. So you want to maximize protection.
On my BMW twin it calls for 20w50 in temps over 50 degrees. I ran Amsoil 10w40 year round in El Paso with no wear associated problems. This was with extended drains as well up to 9500 miles.
In the BMW owners Manual they had a chart depicting which oil grades and temps to use. There was a "special oils" section (which I took for meaning synthetic) that allowed the use of Xw40 in temps from 0 degrees to over 100 degrees. So I felt totally confident using this "special" oil as compared to the standard dino BMW oil.
Lastly when talking synthetics compared to conventional you have a higher leaway as far as viscosity goes becuase a synthetic has a natural higher Viscosity Index(VI) which is the relationship to how much the viscosity changes over a range of temperatures.
This mean if you bike or car calls for xw30 you could use xw20 or xw40 with no problems for the same temperature ranges.
To answer your question a thinner oil should cool better because of the increase flow, unless you are so thin that you come out of hydronamtic lubrication where you would see a rapid rise in overall temps becuase of the friction. Also a thinner oil will carry off the heat from bearing journals and cams to the crankcase/oil cooler.
[ June 25, 2003, 08:28 AM: Message edited by: msparks ]