Originally Posted by
Newjack
I'm not sure what you mean.
The OEM parts on our cars are almost always the best thing you can replace the original parts with. The fitment is factory, the quality is high and you know exactly what you're getting. There are a few parts here and there that are arguably (some factually) better than OEM parts. Things like the G.A.S. coolant cap, G.A.S. DISA rebuild kit, Stett boots, viton gaskets etc.
The expansion tank on the E46 is known to have issues. They fail quicker than they should, because of the high pressure of the cooling system, and the fact that the expansion tank is made of plastic, it tends to crack over time. This is why whenever anyone says "I have a coolant leak and I'm not sure where" everyone suggests the expansion tank first. With that being said, there aren't many other options out there for non OEM expansion tanks, and as long as you replace your expansion tank preventatively, you shouldn't ever get stranded on the side of the road.
The rest of the OEM cooling system parts are fine and I would highly recommend using OEM parts primarily because if a cooling system part fails, your engine can potentially overheat, and the M54 does not enjoy heat at all. The heads will warp very quickly under high heat without proper coolant.
Things like washer fluid pumps, tail lights, anything aesthetic or not critical to the motor or trans are usually what people consider to be fine when replacing OEM parts with non OEM parts. If you buy non OEM front control arm bushings and they fail prematurely, your motor won't explode and your trans will be fine. You will just have to enjoy a shitty driving experience until they get replaced. No biggie.
At the end of the day it's your car and you can do whatever you want to it. Not everyone is made of money and sometimes we have to cut some corners. I'm just giving my two cents about what corners are the least destructive to the well being of the car.