Do you think someone put oil in the transmission? I don't really see how engine oil would make it's way into the transmission since the only connection to the engine is the flywheel. The fluid might just be burnt. Often times, if a gear simply doesn't work, fluid probably isn't the issue. I could be the clutches or bands are bad. It is difficult to actually flush out the whole system without equipment.
Make sure whatever fluid has drained is done draining. Put on the pan (I'd suggest a new filter) and fill it with fluid. If it works, great. If not, then it would need a trans overhaul. I had a RWD car done once and it was $1400. Not sure if there are any salvage yards around, but it might be considerably cheaper to buy a used one (if you can find cheap labor to replace it).