I have never heard of one of those valves sticking before as the spring tries to return it. I would definately polish the spool and polish the bore too. Use very fine emory !! Wrap it around a dowel to clean the bore.
If it moves freely by finger, it should move freely by oil pressure.
The spring strength controls oil pressure so you dont want to touch that spring except to replace it with a new one of the same.
The same oil pump is used on the VW D20 engine which was used in Audi 5000 of the same period and of course the D24 variants.
Check on Ebay for Volvo diesel and Audi diesel parts and you will find stuff for these engines amongst the other stuff. Bear in mind that some parts are in common with 4 cyl VW diesels but only a limited amount.
Steve
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