We've had several Astro vans at work -- they all threw a rod before 200,000 miles.
O-rings die of old age -- it wasn't anything you put in the gasoline. They need to be replaced every six years or so, along with any and all vacuum lines.
My old (92) current work van has 282,000 miles on it, but we change oil every 3000 miles or less, and use good oil. These are usually driven several hundred miles a day, seven days a week, too. This is fairly unusual for a GM -- they usually crap out about 150,000.
If you are burning oil, I would definitely recommend valve seal replacements however much of a pain it is. The oil ash eats rings and cylinder walls. The drawback is that you will find that the valve guides are worn out and the valve springs shot, and will need to do an overhaul. When the heads are off, you will also find the cylinder walls shot, the pistons bad, etc, etc. Bearings are already marginal, as they aren't up to snuff from the factory.
Listen carefully -- if you start to hear "diesel" sounds from the engine at low speeds, you have a rod bearing knocking and the rod will be out the side of the block shortly!
If you have oil in the air cleaner from the crankcase ventilation, get rid of the motor -- they go fast once the blowby is too much for the PCV system.
I'm still not impressed with the new GM engines -- a decent MB will still have good valve seals at that age and milage, or a Volvo -- had to put a head on my sisters 740 turbo this summer because she blew the upper radiator hose nipple off the radiator on the interstate -- fried the head. Bottom half nearly new, the only other problem was a leak at the distributor seals with 160,000 miles on it.
Oh well, maybe I'm wrong, but I will stick with MB for the forseeable future!
1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!