Some input from a fellow user of an old car. I have a 1991 BMW E30 318i, so also ~20 yrs old. Mileage is at 260,000+ km now.

I used to use semi synthetics like Magnatec 10w-40. The recommended viscosity for my engine at our temperatures is 50 btw. Ran fine, but also occasionally had white smoke from exhaust, and by the time I change oil every 7000km, the level would have dropped from full mark on dipstick to between min and halfway mark.

Anyway at some point a year or two back, I did a full engine overhaul due to other issues. Nowadays, I run full synthetic with no white smoke and absolutely zero change in oil level between changes. Am using Mobil 1 5w-50 at the moment as I believe modern synthetics do have an advantage over regular minerals/semi-syns.