I am by no means an expert, but this is what I would start looking at.
Breather on drivers side valve cover is fine. Not sure what else you have on the opposite side, but if at least a breather that is ok.
Run a compression test to eliminate that possibility.
I wouldn't be real worried about operating temp at this point myself. As long as the truck is allowed to warm up fully.
Any water in the oil? Have you changed it recently?