I've been using Bel Ray 75 wt gear oil in my KTM's since, '89, never had a problem. (I run 125's)
I do use Dextron III in the winter, (when it drops below 40 degrees) to help it shift right.
I dunno if I'd use motor oil, it may not work well with the clutches.
Motor oil is designed to cushion between metal parts, (piston bearings, cam, bearings, etc)
Trans oil and 2 smoke oil is designed to cushion gears, but also help with the fiber clutches.
As for your clutch problems.
1. Are they fully engaging, (no play in the cable or master cylinder, (you can take the oil plug out and lookie-see in there)
2.Are the springs cooked??? No pressure/weak pressure against the plates wil cause them to slip.
3. How often do you replace the oil?
If your riding style is "clutch happy" them your clutch life will be short.
Try a heavy duty clutch package if that's the case.
Joe :ride: