Not sure what day Big Ed is referring to “back in the day”, but back in my early Vincent days, straight 50 grade oil was recommended. And I don’t even go as far back as many others. These days, I go with 20-50. And there are special oils suited for flat tappet motors like the Vin.
I was hardly old enough to have even a kiddie cycle in the "back in the day" I was referring to so I'm not talking from personal experience.
The riders handbook issued by Harper Engineers lists SAE 40 for most brands of oil when used in temperate conditions. I think Kent falls into the temperate summer conditions most of the time if they are lucky. Interestingly they do suggest SAE 50 for the Vacuum brand. For tropical conditions, the list specifies SAE 50 so Steve is correct depending on where you are located.
Richardson also lists SAE 40 for temperate summer. I don't know how many people use straight rather than multigrade these days. The low viscosity of multigrade when cold can make starting easier due to reduced drag and it should allow the cold oil to circulate a little quicker. Straight SAE 40 is also recommended for the gearbox although I use an EP80 or 90 grade gear oil. In the engine I use whatever multigrade I have on hand, usually 20-40 as that is what I use in my other bikes.
I have put 48,000 miles on this engine although I have to say I have recently fitted a new big-end and had a + 0.020" rebore. (It still clatters like a steam roller.
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Some of the older oils had additives that were better suited to older designs. Some of these have been removed as they interfere with catalytic converters found on modern vehicles. I think zinc was one of these, not sure if you will find a brand that still lists that, others might know which these might be.
I can see an "oil thread" looming on the horizon.
The Oil Thread continues
here.