Regarding doubts about if oil tank lagging is necessary , IMHO it very definately is. The oil temp research referred to indicated clearly that the bulk oil temp in the tank was below that required for the oil to perform correctly. In fact , I saw 1 ref that the question was put to an oil company and they advised that a 10w oil was needed for the temperatures seen !!
Personally , I think the reason for low oil temp is the slow oil circulation of the Vincent oil system and the large expanse of the alloy engine.
Using a lower viscosity oil is not the answer , temperature needs to be above 80 deg c at least to evapourate water & combustion deposits off at a reasonable rate.
Even though lots of owners reckon they get acceptable mileage between overhauls , I think the engine is capable of much more durability if oil temp was at the correct level.
Happy ending: I bought a roll of 2 mm aluminium foil / polystyrene from B & Q (£6), stuck it to the UFM with double-sided sticky, and used the other 98% of the roll for its design purpose, reflecting heat from the back of a radiator into the room instead of into the wall. What made it particularly happy was that it is invisible in place, and has survived several tank removals and replacings. It also seems to have a noticeable effect in both applications.
My Rapide acquired in 1964 or thereabouts had the Petrol Tank padded at the bottom and in the tunnel with thick co**** felt. I am sure it was intended and succeeded in reducing engine clatter. It settled on the UFM with a pleasing squish. I am sure it was equally effective at lagging the oil tank. I had no problems with overheating. It was only abandoned after I dented the petrol tank and fitted a £5 replacement.