I guess I was lucky, I bought used, not certified, 2002 BMW with 82K. I changed the oil as soon as I got it home. The oil was like tar, the hot oil Plopped out of the pan, under the cap was visible sludge. The previous owner either used the wromg oil with long intervals or never changed the oil. She was an elderly lady so either could be the case.
Anyway, I put in 0w-40 and new filter. changed at 500 miles. the oil was dirty but it flowed out of the pan pretty nice. changed again in 500 miles. changed at 3000 miles. from 86,000 miles til now knock on wood the engine with 124,500 miles is as clean as a whistle with 7500 mile oci. At 7500 miles the oil comes out transpareent and a little dark as opposed to dark as tar.
The engine runs like brand new but, I realize I could have clogged the oil pump screen. I guess I live right or got lucky!