The two British Seagulls I’ve owned used a 10:1 fuel mix and required hard to find 140W oil in the gearbox, or a home brewed mixture of grease and oil I would cook up to resemble 140W. Mind you, these were the larger Silver Century Plus models with a clutch and the smaller motors might use regular 80-90 gear oil. Not sure. I think the newer models went to 25:1 mix to. Also, if they don’t leak out of the crankcase joint, something’s wrong!
Here’s a link to the Seagull site:
http://www.saving-old-seagulls.co.uk/index.html