Originally Posted by sanoke
Thought I'd make this public so everyone could benefit from Mark's answer.
Two of my bikes are air cooled Yamahas (XJ550, XS850). Both are spec'd to use 20w-40 Yamalube. I decided I wanted to try using Castrol GTX instead to save some money so this past summer I've been running 20w-50 in both. I'm very pleased with the results. Good shifting, very little valve wear, etc. However, it's now coming into the cooler time of the year here and last winter I noticed both bikes were sluggish to start with 20w-40. My thinking is to switch both to 10w-40 for the winter months and hopefully things will be ok. The temp here from now until spring won't get much higher than the 60's so I'm thinking the 10w-40 should work ok.
So Mark, what do you think of my reasoning here?
I started to answer this last night but I had a power outage before I could post what I had done
Anyway, its not that uncommon for people to run a summer oil and then a winter oil, IF! your going to be riding in pretty cold temps. Your reasoning is good! And the 10W-40 I would think would be fine.
I'm not a fan of Castrol GTX, yes many people like the oil, so I guess its a personal thing. Keep in mind that when you go from a 10W-40 to a 20W-50 you will notice a performance loss as well. But using the two weights of oil shouldn't really hurt anything.
Now this is something you didn't ask
I don't ride in the COLD so I just use one viscosity of oil in my motorcycle. But! If I was to use two different grades of oil and a non motorcycle specific oil I would look REAL hard at these two oils,
Shell Rotella-T 10W-30 (for winter) and
Shell Rotella-T 15W-40 (for summer)
There is just enough offset in the viscosity range for the climate difference, but not as much of a swing as 10W-40 vs 20W-50. That's a big enough jump that you will more than likely notice a performance difference between the two oils.