Is it gear oil or engine oil, the smell should clue you in?

I'd first suspect engine oil from the rear main seal, if you have a lower flywheel tin cover then after driving and then sitting for a little while it'll drip out the hole in the bottom.

If you have an original transmission I don't think it has an input shaft seal, while later ones did. Either way it's not so likely to leak unless the transmission is over full or lots of steep down hill driving.

Any leak from the back or top of the transmission should be evident after cleaning, driving, and looking for them.

Grigg

•1951 GMC 250 in the Project Journals
•1948 Chevrolet 6400 - Detroit Diesel 4-53T - Roadranger 10 speed overdrive - 4 wheel disc brakes
•1952 Chevrolet 3800 pickup
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"First, get a clear notion of what you desire to accomplish, and then in all probability you will succeed in doing it..." -Henry Maudslay-