I switched to Rotella Syn from motorex power synt. I've got this same issue now clutch is dragging neutral is hard to find etc on hot days 90+ F. Never occurred before in all the times of running motorex. Also the gear box feels like crap on clutchless shifts too.
Rotella might be a fine oil but it's to thin when it's hot and doesn't shift very well.
Back to motorex for me.