I have an 08 E90M3 with 50k. Bought it 2 years ago with 30k as a CPO with 1.5 years of original warranty also at that time. So far, there has been no warranty work.
The regular warrant expired a year ago, so I do the maintenance now. At about 40k, I did the oil and filter, diff fluid, brake fluid, tranny fluid, spark plugs, cabin filter, etc. Everything but the coolant and that is on the list. Battery is likely near the end of its life and will be replaced probably with an Autozone or other local shop battery with a 3 year replacement warranty.
Shifts well, drives well. No issues yet. I did some light mods, like H&R Sport springs with cut guide tubes and E36M3 bump stops, braided steel brake lines, StopTech Street pads, UD pulley, K&N filter, x-pipe, and Vishnu Procede with water/meth. Thinking about an axle back and maybe an intake pipe.
No issues with the original oil cap. If I did have them, I would replace the rubber gasket seal. After that, I would look at the crankcase vent system. On short distance cars, it can get gummed up. If the pressure cannot vent fully, oil will come wherever it can get out.