soffiler
New member
Bear in mind that 20 weight base stock in a 20W-50 only needs an additive package that gets it to perform like a hot 50 weight oil (2.5 times more viscous, corrected for temperature) whereas a 10W-50 needs to get 5 times "thicker" to behave like a hot 50 weight oil. IMO, if you're going to use something with such a robust additive package, you might want to consider more frequent change intervals.
SAE viscosity scale isn't linear as your post suggests, but nonetheless you're making a solid point. 20W-50 uses 20W base stock and needs less viscosity modifier to hit the SAE 50 spec when hot, versus a 10W base stock, which needs a lot more viscosity modifier to hit SAE 50 hot. Conventional oil wisdom says viscosity modifiers aren't lubricants, so the less of it, the better. I do not disagree with that philosophy.
I am checking my Rider's Manual right now ('05 GS). It calls for 20W-50 down to -10C (+14F). I personally don't ride when it's that cold, so 20W-50 works for me. Actually there's a lot of information on that page. Three straight weights, five specific multi-weights, and two more which provide some wiggle room:
SAE 10W-X Special Oils (X>=40) for temp down to -20C (-4F) and unlimited on the high end
SAE 5W-X Special Oils (X>=40) showing the widest possible range, literally unlimited on the high and low end
IF the R12S uses the same specification (I do not know the fact here) then feds27's BMW-branded 10W-50 is covered by that "Special Oils" category all the way down to four below zero fahrenheit. Brrrrr. Again, the 20W-50 is a better choice for me, all the way down to 14F.