tbm3fan
Old Man with a Hat
I guess it may depend on where you live. I mean, if you live in a really hot climate, you may need to use 10W-40 or 20W-50. I don't need it where I live though. I have always used 10W-30 in my classics and have never had any problems because of it. However, around here, the average summer temps are between 65 F (18 C) and 90 F (32 C). It's rare for the temp to go above 95 and it almost never goes above 100 F. I drive my classics into mid-October, when daytime highs are usually around 60 F and nighttime temps can make me run the heater on cool evening cruises. I don't have concerns about viscosity, but do have some concerns about the additives, or lack of them, in todays oils. I do stay with well known, name brand oils and have been adding Comp Cams break-in lube to my oil changes in my 383 (and my 390).
As I'm posting this, it's 67 F (19 C) outside.
I would love to have 5W-__, in all cars. Damage occurs on start up and the thinner the better on the bearings especially if you start every few weeks. ZDP doesn't worry me too much because in the 50's, when camshafts were spalling, they put 0.08% in. In the 60's engines ran hotter and the oil companies needed an anti-oxidant to keep the oil from becoming too thick. There weren't many back then so the easiest was more ZDP and they moved it up to 0.12-0.14% Today there are more and better anti-oxidants so the extra ZDP could be removed because of that and now it is back down to 0.08% as it was in the 50's when it all started.