|Brand is to a large extent irrelevant - they're all going to be good enough for a season's operation (and after a season, the oil should be replaced without fail). The main criterion is that the oil be rated for diesel service, which is class CD. Our club mechanics recommend 15W-40, which is suitable for outside temperatures up to 40C and down to -15C. In NS waters you could probably run with 10W-30, but 15W-40 gives you a bit more margin and the cost difference wouldn't be worth considering against the value of the engine.
C&C 27 Mk IV