The oil originally specified was Shell Vitrea 69. This pre-ISO number became Vitrea 150, post-ISO standardisation. Current equivalent seems to be Shell Morina S1 (or S2) B 150, available in small quantities here:
**LINK** (no connection with the firm, and not a customer)
It's not going to be a critical choice, however, there's a large gear in the gearbox that is Tufnol, or a different, grey plastic that swells with age. So, perhaps, one should avoid lubricants stuffed with additives, or 'synthetics'. Mine piddled oil out from the gear-change lever's bore, but it's a simple O ring seal.