Cars made after the '80s have far tighter tolerances, so the oil stays much cleaner and that's a huge reason engines last longer. If someone had said their car from any year had the original engine with this many miles during the '60s through the '80s, they would have been called a liar, even though some people had cars with a documented million miles.
If you used full synthetic, you may not have needed the Marvel Mystery oil- I bought a '90 Chevy Silverado in 2001 that had been driven for a couple of years in Michigan and the rest of the time in Arizona by someone who worked for GM, last at the Desert Proving Ground. He used Mobil 1 and when it started to puff a bit of blue on startup, my friend & I replaced the oil seals- the valve covers and heads were pristine. The odometer had stopped working, so there's no way to know how many miles were on it, but if they averaged 15K/year, which is very easy in Phoenix, it had over 150K. I drove it full time in MKE until 2011, then part time after I bought my cargo van, also in PHX. I sold the pickup in 2013. With all of the driving I did, including the return trip when I bought it and the second round trip to/from PHX, I think I averaged over 15K/year. That puts it near the 300K mark and when I would look into the hole for the oil fill, it was still spotless.
At the end of March, 2011, I bought a 2005 Astro cargo van that was used commercially in and around PHX- it had 78K on it and the guy knocked off $800 because his guy couldn't get the AC to stop blowing the fuse. After servicing it for the trip, my friend from GM & I found that the high pressure switch was shorted, so I bypassed it and came home. It now has just under 235K and I have had two issues that weren't 'maintenance'- the catalytic caused a code two weeks after I came home (covered by a dealer since it was so close to the 80K mile mandatory warranty) and in Jan of last year, the flex plate took a hike.
I averaged the cost for repairs and purchase prices for the truck and van without gas and maintenance- I think it came to about $750/year since I do most maintenance and only had transmission and axle seals/brakes/pinion seal done by someone else on the truck and the flex plate replaced on the van.
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