I mix two engine oils for 3 reasons:
1. For my cruiser, it gave the best shift and clutch action, compared to several oils not mixed that I tried a year at a time.
2. Mark Lawrence knows a lot more about oil than I do, and he wrote this:
"I'm sometimes asked if it's ok to blend your own oils. Yes, it is. Oils are all made out of pretty much the same chemicals, and nothing really bad will happen. If I wanted to blend my own oil, I expect I would use something like 25% Motul 5100 10W-40 and 75% Chevron Delo 15w-40, which would get me both the advantages of a lot of diester stock (5100 is pure diester) and the diesel additive package. In the winter, however, I would use 25% Motul and 75% Rotella synthetic, to get the superior low temperature performance. In fact, I just run Rotella synthetic in everything I own, year round: ST1300, DL650, Superhawk, Chevy Silverado, motorhome." (I tried the Rotella Synthetic 5w-40, and it was not as good as my mix for either shifting or clutch action). I also did a web search for people using Rotella T 15w-40 for their motorcycles, and found several that have used it for years with no problems. A favorite dirt bike and street bike mechanic recommended Amsoil 10w-40, and it is available at a local motorcycle shop for about $8.50 a quart.
3. It costs a little less to use a good car oil filter (Purolator PureOne PL14610) and use half Rotella oil from Walmart for my motorcycle, and I think the quality is better than the stock Kawasaki choices. I tried all Amsoil 10w-40 and all Maxima 10w-40 for a year each, and they did not work as well as my mix, for the clutch and for the shifting with my cruiser- its wimpy clutch easily slips too much with an oil that is just a little too slippery, and its old style straight cut transmission gears are easily mis-shifted. The ZZR seems more forgiving for minor oil problems, but it runs a lot hotter and at higher rpms with more compression and will probably not go 300,000 miles plus without a rebuild like my Honda 600cc cruiser.