I am posting this under Chevy because it involves a Chevy. There has long been running argument that diesels last longer than gas motors and their example is OTR tractors. The "problem" with this whole theory is that ALL engines do run/last a LOT longer when they are run constantly. My son works at a oil change place that does a lot a of fleet trucks and he got a 2001 Chevy 1 ton box truck built on a van chassis in for a change yesterday. This truck is used to daily delivery across a few states and averages up to 500 miles a day at times. It had 575,000 miles on it with original Vortec 350 and 4L80 tranny. Engine ran well, had good pressure and was quiet. (my son said he was shocked when he got in it to start it up and saw odometer)  Oil gets changed about every 4500 to 5000 miles which is about every two weeks. (conventional oil too) The driver told him that all the current trucks in fleet have at least 250,000 miles on them.  

Did you see the story this week about the Chevy truck that turned 1 million miles? I'll see if I can find it...  

i agree on what snoman says about the pm ,as longas you take care of yuor equipment itll take care of you,its a philosophy thing, for me.its worked so far!