I have a 2002 OB with EZ30 with over 240k on it. I've had it for about a year and 15k miles. The whole time it's been consuming coolant at the rate of roughly a quart per 500 miles. Basically every couple tanks of gas top it off, doesn't overheat if it stays mostly full. Doesn't use much coolant at all when my wife is driving it around town.
I have a 1999 OB we swapped an EZ30 into, also about a year ago, and I've put about 20k miles on that. The first engine from the original donor car was leaking like a sieve from about a dozen places. The next one seemed to have the normal slow coolant losing head gasket leak and supposedly under 100k miles from a junkyard. Got them to replace that one with one with supposedly 80k miles on it. Same deal with that one, and it leaks oil. The oil leak is not really while the car is running, most times if you let it sit overnight there'll be a couple little puddles of oil under it. We have plumbed the coolant system to purge itself of air and I haven't had to add coolant in a long time, but not really a long term or universal solution.
I just purchased a 2001 OB with EZ30 with 156k on it. Has the same oil leaking issue described above, seller estimated a quart per month of normal driving. He says the mechanic he took it to said it was the head gaskets.
So eventually I may wind up doing the head gaskets on all three of these. Thought about just replacing them with lower mileage used engines but didn't have any luck with that for my 99. I've heard the timing chains last forever in these things, what would be worth replacing while the engine is apart? Water pump, head and valve cover gaskets, spark plugs, radiator hoses, timing chains/guides? I've read elsewhere on this forum that the heads and/or blocks are basically always warped on these by the time the head gaskets leak. Is that true? That changes this from a weekend project to an engine build and waiting for a machine shop to flatten things.