T6 is 5W40, and more than one bike has had issues keeping it in the engine... Regular Rotella T is 15W40, and for some riders, their climate is too cool for that heavy an oil... Rotella also happens to make a 10W30 high zinc (compared to car oil) for Kubota and other small industrial diesel engines...
I ran my 98 Savage on Castrol GTX 20W50 for years without issue. And then the formula changed and the clutch started slipping.
Ran my Kawasaki VN800A on Mobil red top 15W50 until the formula changed, and the clutch started slipping.
Tried all the "motorcycle" oils at one point or another in various road bikes, and the clutches slipped.
Stick to a name brand diesel oil, whatever one your particular machine seems to prefer (each engine has its own quirks). My 2003 has behaved best, and gotten it only decent fuel economy, on a blend (50/50) of Delo 15W40 and T6 5W40. Fuel economy on mine averages around 40mpg, on a bike that should be getting in the 70s (based on my 98, which got in the mid 70s to mid 80s, consistently, using 20W50). It almost got 45mpg on the blended oil (couldn't find second bottle of Delo in the garage, so, used a bottle of T6 that the store wouldn't take back).
I generally don't use synthetics in my bike. By 100 miles, the crankcase is empty, regardless of the brand. That's 2 Slopokis and 2 Kawis in a row that can't use the stuff, for the same reason.
I average 75-100 miles per day, year round. If there is no ice, and it isn't a tropical downpour, I'm taking the bike. Hate driving.
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