I thought Rotella 15-40 was a diesel oil? jeff
Jeff, it's a common misconception. HDEO actually stands for "Heavy DUTY Engine Oil", not "Diesel". The fact that it meets specifications for many diesel applications further leads one to mistakenly consider it a "diesel" oil. It's not JUST an oil for most diesels. Also, there are many diesels for which Rotella does NOT meet the specifications.
Many of us have used Rotella for over a decade, BEFORE it came out with it's relatively new JASO MA specification. It's a very good oil for most motorcycles (even wet-clutch bikes), especially for it's price. I was sure of this as a fact for MANY years prior to the JASO MA spec, because I did serial UOA's, and watched many others doing the same thing with this oil on many different bikes.
As for those that say: I've been running "X" oil for thousands of miles, and it's doing just great....I ask, "how do you know"? Maybe it is, maybe it isn't. Without Used Oil Analysis by someone experienced with your particular engine, it's just a guess. THIS is why oil threads get so pointless...no science to back up MOST of the claims.
Instead, most people just rely upon ancedotal evidence, and, sometimes not a lot of it. I had a Honda Shadow that I ran Rotella in since the 1980's, and my nephew still owns the bike. He has over 85,000 miles on it now, with no engine work. Does this mean Rotella is doing great? Or, is it doing better than some other oil choice? No. Maybe almost any engine oil used in this bike's lifetime would have the bike seemingly running just as good at this point. The point is: one bike's apparent history doesn't mean an oil is good, better, or best. I can only say it was good enough in this bike, ridden in the conditions it was ridden in, to get it to at least 85,000 miles.
The fact is, Rotella 15w-40 is a decent Group II Base stock with an additive package that does well enough to meet JASO MA specs. It's ALMOST a 50 weight oil, missing that categorization by only a couple of tenths of a centistoke...so, it tends to hold it's grade a bit better than many other 40 weight oils, since it starts out so thick. Holding grade is very important in combination engine/transmission setups, as the transmission TENDS to require thicker oil than the engine section, unless some valve-train work has been done that may require either a thicker oil or a specialized additive pack. It's hard to beat it's value, as long as it's changed out prior to shearing out of grade....but, that's true of any oil...some hold grade much better than others.