Richard,
Multiviscosity oils, like 10W-30, are the only way to go these days. They pump more easily at low temperature, yet provide superior protection at high temperature. The 20W-20 must be essentially a straight 20 weight oil, hard to find and not worth looking for, IMO.
My owner's manual recommends 10W-30 for most conditions, and that is probably the best weight for moderate climates. I use synthetic 15W-40 (or maybe it's 15W-50, can't recall exactly right now) and have no problems with it. For a few years there was a lot of concern about 10W-40 and some manufacturers recommended against using it. I think Castrol GTX 10W-40 was one of the few that was considered ok at that time, and I'm sure synthetic 10W-40 oils are fine. I probably wouldn't use a 20W-xx oil unless I lived in a fairly hot climate. There's nothing wrong with it, it's just a bit thicker when it's cold so it takes a little bit longer to pump up to pressure. I'd also stay away from the 5W-xx and 0W-xx oils, mainly because I don't trust an oil that thin unless it's recommended by the manufacturer.
There's quite a bit of info on oils on the net, search for "motor oil viscosity". But in the end, sticking close to the manufacturer's recommendation is probably best. Hope this bit of babbling was of some use!
George Davis
'69 280 SL Euro manual
