I have a 2000 Chevy Express 1ton, 350 engine, 240,000 miles. Trans. is fine, rear main seal has started leaking, got quote from co-worker $250 to replace, have to drop transmission to replace it, wish for good old days when cars where built for the owner to work on them. But as far as used cars and miles, unfortunately it all depends on previous owner, if they never checked the oil and let the warranty lapse then a motor with 50,000 miles could be in trouble. Look for someone that kept records, they usually take better care of their vehicles.