You can open quite a can of worms with that question. At that time manufacturers recommended differing weights of oil for different seasons (30 weight summer, 20 weight winter, 10 weight below 10deg). Multi viscosity oils hadn't been invented yet. I believe in following the mfgrs recomendation so I use the appropriate oil for the season. Others believe, and I can't really call them wrong but I do disagree, that using a multi viscosity oil (10W30) is OK. My problem with using 10W30 in my 38 is that it's a 10 weight oil that has the lubrication properties (not viscosity) of 30 weight when it warms up. 10 weight oil gets real thin when it gets hot. These old engines weren't designed to use thin oil. Having said that, as long as you have good oil pressure I think you'll be OK whatever you use.
