Over the years, I have found that websites (realoem, FCPEuro etc)
are, for the most parts, correct but once in a while it is confusing.

The Japanese are better, they usually try to keep the same
PNs for different applications, simplifying data base and repair.

BMW and Volvo etc tend to have different PNs for slightly different items (such as diff driveshafts for Auto vs Man Trans).
This just makes repair more difficult.

For the cars I have at home, when I am bored lol...I look around, and assuming the car is stock, I write down PNs for common stuffs such as:
- Giubo
- Radiator
- Alternator
- Belt(s)
etc...

This way when I need the info, I have it ready.
It only takes a few minutes to gather info on
your existing car.

For cars that you send with your kids in college,
this (gathering info on PNs) is even more important.

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1998 E39 528i 5sp MT
2006 E53 X5 3.0 6sp MT