Then we had my MG1100 which ran the engine oil through the manual transmission also.
@Tester gives an excellent piece of advice there… Ive come across that…I had to install a ball valve on the drain and force feed fluid UP the drain hole before… No other way to fill the thing…or at least it wouldn’t have been easy. You should be OK though…the plug usually come out OK
Blackbird
Just a useless piece of trivia I suppose but most of the older SAABs use the transmission as the oil sump for the engine with the engine oil and gear oil separated of course.
In the gear oil example, very little drains out if you leave the fill plug tight. Whether it’s a car rear end or the lower unit on my outboards, I always pull the drain first, then I can slip the pan under it before pulling the fill plug. In lower unit example, you pump replacement lube in through the drain hole. Then insert the fill plug before removing the pump hose from the “drain” port. Very little drains out because it’s air locked…
That being said, it’s always a good idea to loosen both first as a check to make sure they will come out 