Hi.....happy new year! I have a 1980's era Case/IH diesel tractor and the manual calls for sae 10w above-10 deg. F and sae 30 for above 32 deg. F. I am having trouble finding sae 10w except at my local IH dealer which I can only get in 5 Gal. pail. I was told (by another tractor dealer) that these are old specs and I can safely run Rotella 15w-40 all year. When I changed the oil for winter, it was about 30 degrees F outside and the 15w-40 looked a little on the "thick" side. I am a little concerned that I will damage the engine. Any thoughts?