Z1OwnersClub GB

Just about to change oil on 1981 KZ1100... manual says se class but it’s pretty out dated, thinking on using Castrol activo semi synthetic 10/40 is this suitable?

Any 10/40 semi will do ( Preferably JASO ).
Never use a fully synth unless it's designed for roller bearing cranks such as Rock oil TRM 20/60.

Definitely agree any semi 10/40 will do the job
(I've been using the Halfords one for years)

Don't be tempted to use 10/40 car engine oil as it is not designed to be in contact with clutch and can cause clutch slip
Cheers
Kev