I bought my 96 80 series at ~190,000 miles, (Jan 1 '04) dont know what PO had been running, but I did one (maybe two) dino oil change then switched to M1 at ~200,000 miles
my dipstick was dropping pretty regularly
I wasnt leaking oil, (I park on concrete driveway) coolant looked good, tailpipe didnt look smokey or smell too bad (although I understand synthetic will not burn but perhaps it was being atomized and blown out the tailpipe as a small particulate doing who knows what to the catalytic convertors)
added oil & hoped things would seal up eventually
changed the filter a couple times, and kept adding oil (engine oil light came on a couple times - by the way checked PCV valve it is good)
did a complete drop and refill with synthetic.... tried 15 w 50... still having to add oil... decided that I couldnt afford to keep putting synthetic in it and went back to dino at about ~220,000 miles
at 230,000 miles now, oil level seems to drop slower with the dino oil, plus I can get a 5 qt jug of dino for about the same price as 1 qt of synthetic.
I am running synthetic in the diff's and xfer case
IMO if you can put synthetics in an not have be constantly topping off, then its worth it. I wanted it to work on my truck, it just didnt.