Yep! Gear oil stinks!Is it gear oil or engine oil..? There is a distinctive difference in smell. If its engine oil, it maybe the upper timing chain cover leaking & running down around the box?
Dam! I should have thought of that one!!! As I had same issue on mine 4 years ago! Still at least you found it.. It's the plug cap to the lower front driver side of box yea? The rubber coating decays with road grit (salt) & eventually eats its way through the metal plug itself.. Look at the bright side, you now have to do a gearbox oil change, so you'll have nice clean gear oil giving you slightly smother gear change.. ????