I have never asked this question before, always went with what I thought was good. 1989 281K PLUS what I think may have been a lot of hours idling to run an 120V inverter when she was used as a locksmith van??? In the summer I have been running 20W50, I find it interesting that when i do run the 20W50, esp right after oil change the tranny slips more often....as the oil gets more and more fuel diluted and beat up/thinned, the tranny slips less, as it does when I run 10W40??...what is that all about? I do have ring issues and fuel dilution, changing the oil about every 2500 miles. orig spec oil is 5W30, will this be way to thin now?? had thought about Rotella T3 10W30?? whadya reckon folks?