Is it doing anything untoward, like blowing clouds of oil out of the exhaust?

Presumably this is the non start fuel injection engine. Not wanting to start is bound to have dumped a bit of fuel into the bores, the black stuff in the oil comes from the combustion process of the petrol, run one on LPG gas and the oil stays eerily clean. I wouldn't worry too much, change the oil and try to run the engine in with decent length runs rather than lots of stop start.

Whatever you do don't run the overdrive low on oil, it is something they are very intolerant of, does the planetary gear pins a power of no good. The gasket between the overdrive and the extension tube is fairly painless to change, the gasket between the extension tube and the gearbox is a pain, once you remove the extension piece the mainshaft can come back slightly, after which the box needs a strip down to put it back together again. Have a close look at where the oil is really coming from, that will indicate the level of work you are looking at, they tend not to be completely dry however.

Regards