I have used the css 20/60 in my tr7 powered tolly for ages.
I suspect the oil is a bit thicker than a GOOD QUALITY, modern 20/50 when hot, but not a lot. However, if the car gets driven vigorously it should maintain pressure better. I understand and respect Jeroens point, it is 100% valid. But many older tech oils just don't cope well.
at 100 degrees the css 20/60 has a viscosity of 22.7, pistoneeze20/50 18.5 castrol classic 20/50 17.3 comma/halfords classic 17.2
Those figures will obviously drop when the engine is hotter, 120 degrees and I would expect the 20/60 to maintain viscosity way better, whereas the old mineral oils would start to thin. That cost me a crankshaft, me thinking a classic oil would be adequate in a 1500 toledo at Goodwood.
As an aside, John kipping used to reckon the BEST answer to keeping a 1500 alive when thrashed was mobil 1 oil. And I don't think that was available as a 20/50, or indeed any 50 weight oil. Which supports Jeroens views, the viscosity isn't an issue (as long as it doesn't fall apart!) but maintaining the integrity is key. And a modern ester synthetic will do that best. HAve also had a friend who has tried millers NT oil, reckons it is the dogs danglies and maintains viscosity very well, better than anything else he has tried in his very quick 2.5 gt6.
Please feel free to take my thoughts with a pinch of salt.... but do think about modern synthetics or decent semi synthetics. Modern oils are just better.
Agreed, though the key word is synthetic, that is where the improvement in oil is found from the 1970's Castrol GTX formula, maintaining it's viscosity at temperature as well as prolonged life meaning fewer oil changes.