Some here claim that from the same manufacturer OEM is sometimes different than aftermarket. I.E., on my Patriot the OEM Akebono brake pads are completely different than the Akebono aftermarket. If my plans are for long care ownership, I have joined the camp of OEM on many (not all) car parts.
With German cars, just as with auto glass, there are three grades. OE, OEM, and aftermarket.
Folks who are in the Japanese car world often use the term OEM, to mean OE. Which doesn’t help matters. Also, on purpose, it’s not transparent.
A $16 Bando on Amazon, which I have in 2 vehicles, is not OEM. It’s aftermarket. And good enough, there was no need to spend $60-$80 on OE (there are Mitsoboshi and Bando OE p/n’s).
With BMW and GM from experience. The difference between OE and OEM is packaging and stickers. But we know going into it. With Japanese, we do not.
Denso is OEM to GM. The factory GM OE alternator on my wife’s car is Denso, made in USA. Her car was mfg in Michigan.
Denso, AC DELCO, Valeo, these cos are OE, OEM, and aftermarket, all 3.
A $100 Denso radiator is not the same as a $432 Denso from a dealer. Nobody claims that it is. First is aftermarket, latter OE. In this case you do want OE. With the belt, you do not.