It operates between 50/60 degrees and is used on a yacht.
I'd presume this is a diesel, as found in many Morris and Austin vans, in which case I'd use a diesel 15W-40 and change it every 100 hours or so.
It is a fact of life that it takes more energy to pump more viscous oils around the engine. In the drive for improved fuel economy it is now common for lubricant viscosities as low as 5W-30 and even 0W-30 to be specified for car engines. The disadvantage of these low viscosities is that separation of lubricated components is reduced and wear increased. Modern car engines overcome this by hardening crankshafts, making bearings from various aluminium alloys and other means. This is not the case with yacht auxiliary engines, which universally use traditional metallurgy as does your BMC 1500. Oils with viscosities of less than 15W-40 are thus inadvisable.
though modern oils are extremely effective in modern engines. however.. they do lack some vital elements needed to stop excess wear on older components not designed for modern lubricants. hence classic and vintage cars do NOT use modern synthetics.
some poor reactions have been reported with seals too ( leaks etc) so ideally we would want an oil for low temperature operation and designed to suit an older engine. w scan get recommendations from class car sites. or if we contact the oil suppliers technical department we will get a suitable oil from their professional services to sit our requirements.
eg Castrol classic oil. likely normal car spec is not correct for marine use as more intermittent and much colder running.
This engine would certainly not have seen 'fully synthetic oil' in it's day and you may find it will simply burn it or it will find 'leaks' you didn't know you had !
Lots of these old BMC engines (petrol and diesel) found their way into marine applications - often found on canal boats, so finding some 'specialist' advice as suggested by 'racingfan' may be your best bet.
You can't go wrong with 5w-40 fully synthetic. Asda and Tesco sell it at half the price of Mobil but it is to full specification.
Castrol GTX is good in BMC engines.
Check with your local marina and see what they use.
It is a 1500 35hp dating to 1980.
It operates between 50/60 degrees and is used on a yacht.
