Later Volvo owner's manuals in fact do recommend synthetic oils for heavy-duty conditions. Whether you will gain anything in an older high-mileage car by switching to a somewhat superior oil is questionable. However, all my vehicles use Mobil-1. All were changed over at various mileages up to 260k mi. No new leaks or increase in the existing ones, overall slight decrease in oil consumption. I observed notably faster cranking and oil pressure response (mechanical VDO gauge) in cold weather - the toughest time for an engine.

I've experimented with various viscosities, but now just use 5W30 year round, as where we live we can get very cold winter days but rarely over 85F in summer.

But for the vast majority of older cars (not just Volvos) you're more than safe using the cheapest good quality oil you can find, a good filter, and changing it in accordance with the owner's manual recommendations.
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Bob (son's 81-244GL B21F/M46, dtr's 83-244DL B23F/M46, my 94-944 B230FD; hobbycar 77 MGB, and a few old motorcycles)