Another couple of corrections needed!
British Seagull used to sell SAE and EP 140 and 90 oils. They are safe to use.
The myth about yellow metal and GL oils I thought had been put to bed a long time ago. Seagulls do not care what grade the 140 is for non sealed boxes, nor, for the sealed boxes, what grade the 90 is.
They do not suffer from and never have, any corrosion on the bearings due to oil.
Seagull knew their stuff, it is only ever a problem in 'hot oil' applications.
So unless you are running a Seagull loaded withGL5 EP140 over a very hot spring, they care not, they are super water cooled and will enjoy the oil, far better than no oil!
I have not come across oils in normal use that do not emulsify, but if you buy an oil elsewhere, use it for a few hours and check. If you see it has gone coffee coloured, all is well. If not check for water in the box and if there is any, change to a different oil.
My advice to all with gearboxes, on ANY Seagull, is to only fill half full. If you lay a motor down and fill to the top or nearly to the top, when it is stood upright, lashed to a pole or clamped to a rail, for storage or in use, half of that oil will eventually dribble out into that drip tray! Messy and a waste of oil.
Sadly British Seagull seem to have sent out mixed messages about this over the years. They did say fill near to top at first, then modified it to say the gbx plug was a combined fill, drain and level plug. In the latest version of the workshop manual they have made claims that the 'gearboxes on all Seagull are near oil tight!' Not true. (They have also got the plug gaps wrong and the advice about clearing water blockages. so I have to add adendas to new manuals, not good!). I am not certain how much time anyone at Seagull have spent hands on a Seagull.....
I suggest that after a motor has been in use for decades the bearings will be worn and so check the oil every 5 to 10 hours use. I have never seen an old Seagull that did not leak oil!
If emulsified and no water droplets seen, top it up. If any water drain and refill.. Far, far cheaper than replacing bearings and cogs... if indeed any are still available!
Air safety rules stop oil going air mail these days, but in most countries owners have managed to source 140 oils cheaply. for UK owners I sell it £5.50 a litre. plus post.