This is my experience:
I bought the Volvo Diesel back in 1992 and overhauled the short block. Up until the beginning of this 2009 I used regular gasser Oil it it.

I had 3 gas cars and 1 Diesel and wanted to use 1 grade and type of Oil on all of them; and I did. 20-50wt dino is what I used.

I also change my Oil about every 6-8 months and am a low milage driver.
Also, for the 1st year I owned it the Mercedes got the gasser Oil.

This year I switched to Rotella T for the Diesels and I only have 1 gasser.
Also the Volvo does no soot up the Oil like the Mercedes does.

I would have no problem using up a 2 cases of good quality Oil.

Maybe you could just save it for winter use.

Some members have mentioned that the synthetic for Diesels quieted there valve lifter tick in their Hydraulic Valve Lifters but not all said it worked for them.

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84 300D, 82 Volvo 244Gl Diesel