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.