You must be using some oil if it's turning the oil black in 200 to 300 miles. My guess is
the rings are shot. "had been rebuilt recently" could mean a lot of things. I've seen guys
that wash the grease off, shoot some paint on the engine, give it a tune up and to them it's
been restored or rebuilt. I've seen guys that just run a hone up and down in the cylinder a
little and put a set of standard rings back in even though there was a ridge and
considerable wear. I've seen guys that simply use a valve lapping stick and some Clovers
lapping compound on the old valves and call that a valve job.
A proper rebuild would have the block and head either hot tanked or baked. 50k to 60k
miles on these engines and most would need the cylinders bored and fit to new pistons,
crank reground and new bearings fit, new cam bearings installed. In the head, the valves
guides should be replaced, the valves and seats ground and/or replaced and springs
tension checked and adjusted.
Then driven regularly your 235 should go 3000 to 4000 miles before you need an oil
change.
Denny G

Denny G
Sandwich, IL