Years ago when I was riding a Har....... (sorry) I asked what I should use if I couldn't get the official HD oil. The Factory view then was that any similar grade car oil would do if it was formulated for diesel engines. HD engines are similar to Vincent engines in respect of size, configuration and rev range. With this in mind I have been using such an oil for many years and about 35000 miles without anything bad happening yet.
What you use is up to you . . . you pays yer money and takes yer pick. 15/30, 20/30, single grade, it doesn't seem to matter much unless you are racing. But if you are racing you'll know a lot more than I do about lubrication.
Controversial subject but there r particular characteristics of the Vincent lubrication system that need awareness. Oil circulation is very slow ,due to the type of pump and the oil does not attain a good working temperature. Therefore high viscosity oils should not be used. With modern lubricants , SAE 40 would be a good choice to give the most rapid circulation from start up and allow the pump to provide a good circulation rate when at working temp. Go for a brand that has a recent SAE classification , I think the latest is SM , but nothing older than SH.
When my father and me started the Vincent thing we drove 15W40, the good old Castrol GT3! We changed it not too early, but everything went well. A few years later I used fully synthetic oil from the very best. After thinking a while and compare the problem with my aircooled car I came to one solution: ball bearings, rubber washers, wires in the rocker bolts, all communicates. The oil lines, the need of oil was designed by Irving with the quality, viscosity they had. Today, we think about e.a. paper oilfilters, they have an extremely lower resistance, wires in the rocker bolts, the diameter in the rubber washers (timingside) all communicating oil lines are more or less fed with oil! With every change you will change the oil system. Modern 0W50 or 60 oil needs a steady flow. In a Vincent oilline this flow will break together, if the viscosity is not able to work together with our “ejaculating” oilpump. In my point of view we must find the best way between modern oil and the viscosity Vincent/Irving where using. 20W50 is now what I prefer. You can find this oil “legiert” (sorry don’t´t know the words in english ) modern, or old fashioned. You will find what you need. It helps you during warming up and is able to support the oilsystem our engines need.