First, this is an excellent question and one that I often grapple with. I currently own a 2001 and 2003 Outback and have removed and repaired almost every movable part on these cars; Front and rear struts, ball joints, inner and outer tie rods, brakes, rotors, center differential, most front and rear bushings on both cars, alternator bearings, rebuild power steering pump, engine rebuild down to splitting the block, etc. OK, now that you know where I am coming from, here is what I do:
For most of the parts, I use OEM parts for several reasons. First, I tell myself, without any evidence, that the manufacturer knows how the cars wear and OEM parts are likely to have been updated to match known issues. The bearings in the center differential are an example of this kind of update where the thickness of the race has been modified significantly to deal with a known problem. The second reason is fear of counterfeits and the wide variety of quality in aftermarket parts. I have no doubt that some aftermarket parts are far superior than the OEM parts but I have also, far too often, dealt with aftermarket parts that have been manufactured to fit many applications but actually fit none of them as well as an OEM part. And if you search for information about counterfeit automotive parts, you will find that almost everything that is sold by a big brand name, e.g. NGK, SKF (bearings), Bosch, etc., has a counterfeit counterpart. The third reason I go with OEM is because I shop online and often the cost of the parts online are not horrible, even compared with aftermarket parts. An example of this is with a recent strut replacement. I bought OEM struts for my 2003 Outback. The struts were marked KYB so I asked myself how much I would pay for KYBs purchased as aftermarket. I found that the price of KYBs at Autozone were about the same as what I paid at https://www.subarupartsdeal.com/. Struts could be found for less on Amazon but I have a high fear of counterfeits so I only compared the price against the price at a big chain. By the way, I typically have time to allow a car to sit if I have to order a part so I avoid buying parts from the dealer unless it is an extreme emergency. My local dealership typically charges anywhere between 50% to 100% OVER MSRP for parts, especially small parts, and this is not atypical. Finally, since I am performing the work myself, I am already saving a bundle so I don’t feel so bad spending a little extra on OEM parts.
Now some specific comments:
- I would probably buy aftermarket bearings if I could be sure of the provenance (where it came from)
- Aftermarket rotors are probably OK but the selection has always confused me.
- I like to buy the OEM brake pad kits because they come with new retention springs and anti-squeal grease
- I use OEM oil filters. I used to use aftermarket but was convinced that the OEMs are better. Online, I pay $4.67 each for OEM so why not. When I order, I usually buy a number of them, 10 or so, and buy new drain plug crush washers at the same time.
- Buying new axles is painful. I have purchased a new axle and a remanufactured. The re-man had the hole for the retention pin drilled off-center. I will not buy re-man again. I don’t know if I will buy OEM or aftermarket. One of my cars has aftermarket and it seems to be OK.
Sorry if this does not completely answer your question but I hope it helps.