If you 'think' it may be engine oil because if the colour then its probably OK. If engine oil has been used do not start and you can remove pan to minimize contamination. I am guessing its been run. There are flushing machines but how successful I don't know. Some seals can swelll with different oils. Clutch material becomes contaminated and can give error codes due to wrong amount of slip. The computer is matched to the fluid on modern transmissions.

mags
July 2011