Please don't use synthetic oil in this engine. Synthetic oil is ment for modern cars with very narrow tolereances. If you use this oil in your Rapier you risk dirt coming lose and this dirt is able to block your oil canals and on top of that you risk a lower oil pressure because this oil is relatively very thin. Use a classice oil 20W50. I use Castrol but there are other brands that are equally good.
Agree with Peter. We sell oils from our warehouse and regularly get people with older vehicles, tractors, trucks and cars asking the same question. We never recommend a 'newer' spec oil as it is full of additives for the newer engines which require high mpg and lower co's. and as Peter suggests is too 'thin' in viscosity. I have three classics at the moment and vary the lubes between a good quality 20/50 or a straight monograde oil SAE30 or 40. These oils are blended for use in the very engines we have in our old cars. The only thing I have ever used a synthetic in was our 1040cc Imp engined sidecar racing outfit, when we used Castrol 'Edge' 10w60. This was recommended by Castrol at the time and proved very good under racing conditions at 9000rpm. Hope this helps.
Not been on for a while, but just going to post up some National pics later & saw this. I agree to keep to mineral oil. I always used Duckhams 20w/50 but they got taken over by Castrol (I believe?) & they don't seem to make it anymore, however, I think the 'Halfrauds' Classic 20/50 is Duckhams & I have been using this in my Rapier, has same smell as the 'old green stuff' & I find it pretty good for £19.99 and sometimes on offer as 'buy one and get one half price'. Cheers. G