15W/50 is NOT like straight 50W. Straight 50 is like syrup when it's cold. To put it simply, or at least how it was explained to me, was that multi viscosity oil, the first number is basically the Winter or cold viscosity, with lube qualities of the heavier weight when the motor reaches 212*. For the oil to actually get to the 50W viscosity takes the high temps. This is done with "viscosity modifiers". The cold or Winter viscosity number is based on how hard the oil is to pump through a given orifice at low temps and has to meet pumping pressure requirements based on temps and test orifice size. So what does all that mean? Simply put the 15W/50 oil MUST pass SAE J300 standards of a straight SAE15 to be labeled as such, as well as passing the 50W SAE J300 standard at 212* So that said, based on average dyno room temps and his bearing clearance, no way should 15W/50 be too heavy when cold. Factory motors used to run as tight as his and straight 30 was what most everything came with back then and they weren't grabbing bearings, even in extremely cold temps.

I don't know what caused his issue, but I highly doubt it was the viscosity of the oil, the lack of full groove bearings, or similar.