Pratap a piece of standard 3/8" copper pipe I have here has a wall thickness of .75mm giving a bore of around 8mm.
I think the routing of the Matchless oil lines leave a bit to be desired, but the G3L pipes would all have been produced to the same shape, which are hampered by the inboard foot rests. They loop round behind the gearbox to give clearance for adjusting the primary chain, But I have seen them being fowled by the gearbox adjuster when I guess the chain is at the end of it's life.
The oil lines have male fittings to the engine and one male and one female at the tank.