I'll agree that the oil was the culprit Steve. Back in the 80's when I had my Kawasaki Z900 I got sucked into trying Valvoline XLD. I was on a military logistics course at the time, and I had to ride the 200 miles home on the Friday night, and back to the training school on the following Sunday afternoon.
I did an oil change at home on Sunday morning, and took off around 2 pm. I'd noticed that the gearbox felt a little more "notchy" than previously, and the clutch didn't "slip" when I fed it in and out, like it usually did. It was either "on" or "off".
Anyway, at the 100 mile mark I refilled the gas tank, and had a donut and coffee. I finished my snack, and hopped on the bike and hit the button. I "clunked" it into 1st gear, (it usually didn't clunk at all) and went to dump the clutch, and it stalled. Bummer.
I pulled the clutch in, hit the button, went to take off, and same deal, stalled it. Fcuk. I finally fired it up, revved it up to about 5K RPM, dumped the clutch, and wheelied down the road, woohoo!
Next weekend I dumped that Valvoline schidt out of my engine and just put in some cheap Mobil oil, and it was all good again. I've never used Valvoline since. (I think I've used it in a car engine and it was OK though) Cheers, Terry.