aja8888
Moderator Emeritus
I knew someone with a small fleet of diesel trucks for their business. These things had large oil reserves, so an oil change was big bucks. He'd send the oil in for testing, and found he could really stretch the oil change interval (I think the trucks had hour meters on them, rather than go by miles, since they'd spend time idling at the job?) before the tests started indicating the oil was losing effectiveness (contaminants, viscosity, acidity, etc).
-ERD50
Besides viscosity and metals, we used to look at TBN (additive package remaining) to get a read on what's left of the oil.