Well of course they are designed to do that, they used a similar oil system to the Triumph Bonneville. If it isn't leaking, that means there isn't any oil in it! Foolproof.
Still prefer that solid UK engineering to paper-thin American things. Ever seen a 757 after a heavy landing? The upper fuselage skin resembles the surf at Waikiki. Just ask the spotty M boys about that!
The J41 was a pretty good performer from memory, but it lost some of the that edge when they re-designed the cowlings to stop the engines shutting themsleves down.