Ten years at least, with correct maintenance.
The S class is a great ride esp. on long trips. I don't mind tooling around town in them either -- panzer-like build quality.
When I was at the gas station last week I was surpised to see a "soccer-mom" with a '90 560SEL, 230k miles, decent shape, just needed some cleaning up.
She told me that she was in a terrible car accident, wanted a super-safe car that could haul her kids (I noticed one strapped in the back seat

) and refused to buy a mini-van or a Volvo wagon.
Chalk one up for the 3-pointed star!!
Anyhow, she was thrilled to be driving such a safe, powerful car, and she spent under $9 grand.
Oh and about the Olds, the mileage isn't surpsing. Build quality isn't too great, esp. that era, but the motors are durable. I've owned 2 that have seen 200k miles with no trouble. No comparison to MB, different animal. Have an '83 Delta 88 that I use for a 3rd car -- for the malls, crappy weather, etc. Total POS, but I paid $500.