Now repeat after me: "The first number in a multigrade oil rating corresponds to the cold start-up viscosity ('w' stands for winter) while the second number corresponds to the operational viscosity". Sooooo, if the manufacturer lables it properly, a 0w-40 will start moving around the engine on start-up easier than a 20w-40. Both oils are still forty-weights once the bike is up to temperature, however. As long as you use a forty-weight oil, that's all that really matters (if it ain't too cold out). Bikes usually don't need too low a start-up viscosity given their normal usage. I run Mobil's Delvac 1 5w-40 oil because it is a great oil (that I got a hell of a deal on) that will lubricate the top end a lot sooner than 15w-40's or a 20w-40 when I start it up on late Fall mornings. I'll run it in single digit temperatures as long as it ain't freezing out.
I don't see where a fifty-weight would gain you anything in an FJR or similar engine. Liquid-cooled multi-valve engines are better served with forty-weights. These oil circulate faster and remove heat quicker than thicker oils. I would expect that a fifty-weight oil would increase the operational temperature of the FJR beyond that of a forty-weight. Slight pumping losses would also occur. H-D's, etc. are air-cooled and therefore have looser tolerances to accommodate heat expansion. A fifty-weight may be required there. My friend used the Delvac 1 in his Buell and the engine started consuming the oil. Obviously that particular forty-weight (and likely most forty-weights) were not the answer for that engine. An argument might be made that fifty-weight viscosity would have further to shear over the same time. That, I expect, depends upon the oil more than anything else but wet-clutch applications are all hard on the oil viscosity. Don't leave it in there too long.
I like synthetic heavy-duty engine oils (HDEO's) in 5w-40 grade. Shell Rotella T synthetic is one of the better deals for bikes. Esso's XD-3 0w-40 is another great oil that is currently available only here in Canada. Non-synthetic HDEO's are just fine, too, if cold start-up ain't a concern for you.
Hey, what's a bike forum without an oil thread or three? Wanna talk about filters? I'll go away now...
John.
