I have a 1990 Sedan Deville with the 4.5L. The car is quick, and responsive, so I figure the engine is still in great shape despite 283000 K on the odo. But when the engine has been sitting for a few hours I get quite the knock on startup. I bought a 91 4.5 for parts (I hit a deer and needed a fender and hood). It had 450000 (!) and still ran mint but it also knocked on startup. Is this normal for these engines? I run 10W30 Valvoline oil. Would switching to 5W30 alleviate this knock? It really sounds unbecoming for a knocking roar on startup in a Cadillac.

Do a search for "Main Bearing Thump". That is more previlant on a 4.9 I believe but the 4.5 is just an upgraded version of the 4.5 and with the miles you have that may be a viable explanation. If anything, you'll want to go to a 15W40 Rotella oil, not a thinner 5W30.