Great idea on the oil analysis. Though we are teetering on being a little anal.
I managed a pretty good size fleet of trucks and industrial equipment back in the 90's. All this stuff was run under extreme conditions, lots of dirt/dust and very hot summers, 115 degrees was common. Most all of it was diesel equiptment that saw heavy load use 70% of the time and the rest at idle.
I did oil sampling on all the different fluids we ran, engine, trans, hydraulic, yada, yada from a independent outfit. (don't send it to oil companies!) What I found was we toss out a lot of very good quality oil in this country. The stuff lasts a lot longer than anyone would imagine. I ran mostly Shell oil products at that time and it stayed good for at least twice as long as I'd of figured. The additives and such stayed in the oil, but where we had an issue was contaminants getting into the oil. Not necessarily dirt/dust, but engine blow-by. It ended up I changed my oil change interval from 100-150 hours to 300 hours. 100 hours at 50 mph is 5,000 miles. 300 hours at 50 mph is 15,000 miles. The Rotella 15-45 engine oil dino oil I used still showed pretty good at 300 hours, but between 300 and 400 those contaminants started to show up good. On some of the machines I'd change filters between oil changes, and we ran Lubrifinders where we could.
I think 5,000 miles on a Jeep with a quality syn oil is a good interval.
BTW, yes I know we don't average 50 mph in our Jeeps for 5,000 miles. But with a lot of industrial equipment you do run at higher rpms/loads compared to a on-street type vehicle.
2006 Solar Yellow Rubicon Unlimited
Jeep Club Member #1340
6 Speed, Hardtop
Mods done: Hurst tee handle, cheap hand throttle, Rokmen Merc front bumper, Warn 9.5 TI winch with 3/8" X 100 worth of Viking yellow rope, DPG OME Ultimate with JKS ACOS up front, Kilby Gas Tank Skid, Kilby Steering Box Skid, Jeep Medic Belly Up, Skidrow Engine Skid, Rockcrusher Diff Skid in the rear, Warn Diff cover in front, , AR Outlaw II's and MTR 12:50/15's, Homemade rear Bumper, Cheap Cobra CB, Puma OBA, Sirius Radio, Locker Defeat, Rockhard cage, Rockmen short corners, homemade tire swing/tailgate hinge affair, Airlift air bags on the rear- - - - - - and more to come!