Latest API SN spec for engine oils calls for slightly reduced amounts of Zinc and Phosphorus (abbreviated to ZDDP) additives to preserve the efficiency of the catalytic converter.

This is not a real issue for the OHC Volvo engines as they have large (wide) camshaft lobes and spring pressures are not great. (Cam/lifter wear is an issue in pushrod engines with flat tappet valve trains. Classic car catalogs often offer a ZDDP additive for those old engines).

The red block Volvos are pretty tough so you may be able to nurse it along for a while despite the rod knock. If the oil your shop suggests meets API specs for the year of your car (probably SG) or higher, it will be fine. And yes - stay away from additives. They risk messing up the chemistry that all those refinery engineers designed into the oil.

It would be a good idea to check your oil pressure with a mechanical gauge to be sure it's up to spec. The fact that your dash light goes out just means you have something more than about 10 psi!
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Bob: Son's XC70, my 83 244DL, 89 745 (Chev LT-1 V8), and 2010 XC60. Also '77 MGB and some old motorcycles