83 1100E oil 0/W 50W synthetic

[Nessism]

I believe I read or was told manufacturer's can run tighter tolerances with synthetics due to synthetic oil molecules being all the same "size" as compared to conventional oils which are of many different sizes.

Incorrect. Whoever told you that doesn't know very much about bearing design.

[Guest]

Back in the '70's and '80's, ..., I pretty much stuck to Castrol GTX 20w-50.

Not a 'bad' oil, and the higher viscosity is great for generating pressure. Just remember that it's not pressure that lubricates the engine, it's volume.

Imagine for a moment if the oil passage just downstream of the pump and the sending unit for the pressure gauge was completely blocked. The reading on the gauge would be sky-high, but there would be NOTHING flowing through the bearings. It is far better to have an oil that is thin enough to flow through the thin clearances of the bearings. Fortunately, most GSes have roller bearings on the crank, but still have plain bearings on the cams.