My 1990 G20 conversion van spits out a bit of smoke upon startup. I realize this indicates an oil leak around the seals (right?), which is common in higher mileage vehicles. Once it is running, there is no smoke.

My question is - what is the best way to treat this to prolong the life of the engine - I would like to make this van last as long as possible, obviously. Do I have the seals replaced (how expensive is that?) or is there a treatment I can add to the engine oil to temporarily help this situation without doing more damage?

Valve seals and guides are common issues, you could have just the seals done about a days work on a van, but if the guides are worn this will not work, the heads have to come off, no oil additve can correct this.

My 87 Chev G20 van with the 350 does the same thing. A pretty puff of blue when starting. Im sure its the old valve stem seals and some wear on the valve stems themselves. I change the oil about each 4000 miles (nearly all of our mileage is on long highway drives) and it uses a quart each 1800 miles. The original unrebuilt engine has 248,000 miles on it, though only 50,000 in the 6 years we have had it. To fix it I will wait until something else requires pulling the heads.

worn valve guides or stem seals. You could get a set of low mileage junkyard heads.

A little oil goes a long way. How much oil are you actually using between changes? I had a Ford van that would smoke a bit at startup but it didn't burn even a pint between changes. Unless it is burning so much that it is affecting performance or creating problems with the O2 sensors or the catcon, I'd probably just monitor the oil level closely and let it go at that.
FWIW
YMMV