I've been pondering this myself recently. 20W-50 was original specification, but getting a good one in that exact viscosity might be tough, although I see Valvoline make several. There's Mobil 1 in 15W-50, although not readily available here apart from grey market importers from the USA like GL Lubricants (see eBay or their website). Shell make a semi-synthetic in 15W-50 that's about half the price called Helix HX7K that any Supercheap Auto would have, and it's probably better than the old standby Castrol GTX 20W-50. Really, I think just about any modern oil is superior to the stuff they were using 30 to 40 years ago, so if the viscosity range is right, I would expect it to be just fine.