Paranoid new buyer question
 

While adding some oil to my N62 last week, I noticed that the filler cap was askew. It wouldn't turn enough to be "square" in its place. I also noticed that there was oil seepage down the top of the valve cover below the cap, blow by pressure must have obviously pushed some oil out. I ordered a new OEM cap from a BMW dealership in SF on Amazon. Put it on today, a quarter turn, fit squarely.

It made me think, is there any chance that my seller installed an ill-fitting oil cap to reduce pressure that might reduce Valve stem seal seepage and the resultant smoking at idle during my pre-purchase inspection? ...Or do I "know just enough to be dangerous?" Thanks!

2008 X5 4.8i 167k miles

That is pretty paranoid.

Wouldn't a bad oil cap cause CEL?

Poor idle?

I think the crankcase is under vacuum and not pressure.

Nope, minor fluctuations due to pistons going up and down but the combustion process creates positive crankcase pressue.