Something doesn't sound right there!! I've been using Rotella "T" synthetic (okay, "semi-synthetic" for the oil-****'s out there /forums/images/graemlins/tongue.gif ) in my Deeres (Yanmars) as well as the 310D loader/hoe, for several hundred hours, and extra heat is certainly the opposite of what I've experienced. I'd look for another issue.
<edit> Oops. I don't think they make a 15-40 synthetic; double-check your jug - it should be 5-40. </edit>