Well after about half an hour with speed up to 30km/h, the diff was slightly warm so I decided that will do.
As soon as I undid the plug I remembered why you shouldn't run engine oil in a gear box. The oil was all milky, not from water but air. Engine oil has detergent to wash the bores but that also lets it foam. I don't think I would have done any damage going for a short run but I'll remember now to only use engine oil in a diff or gearbox in an emergency.
I'll post some pics of the oil colour soon.
Happy days

