OEM spark plugs are good for 60K miles. Replace yours, it is not hard to do. While fuel filters are scheduled to be replaced at 90K miles, that should not be your solution to the hill problem.

Did you look above the RR wheel arch? Just to the right of the gas tank? Access should be a removable plate in the cargo floor, under the carpet close to the rear seat back.
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My name is Klaus and I am a V♂lv♂holic

I know zero about the V50's, but basic wisdom says that if you are losing power when climbing hills that you should suspect a fuel delivery problem - weak fuel pump(s), clogged filter, flaky fuel pump relay.....

If the fuel injector rail has a test point valve, you might try attaching a pressure guage and taping it to the outside of the windshield so that you can monitor it while driving. I recently did that on our S80 when trying to diagnose a bad fuel pressure sensor. Hey, there's another possibility - when the fuel pressure sensor started to fail, it was first noticeable when accelerating up uphill onramps, etc. (when fuel demand was the greatest). It had to get pretty darn crappy running before it finally triggered a check engine light and fault code.