[Gelbster]

A couple of simple suggestions ,but I am not an expert:
1. saturate the fill plug with PB Blaster or better penetrating fluid for hours. Do not use excessive force to remove it -just reapply the penetrating fluid.
2. with the drain plug removed and all the fluid drained ,insert your finger in the drain hole and try to feel for a disc shaped magnet. Gather up as much of the 'goop' on the magnet and from the bottom of the case to examine.
Lots of goop means the seals on the main bearing have failed and the goop is the grease that should be in the bearing - yes the same fool who specified the IMSB also did the bearings in the gearbox? The bearing will fail because of inadequate lubrication because it is above the level of the fluid.
Wash the goop with solvent/gasoline and look for shiny gold colored flakes and particles. This is your 2nd gear dog wearing. Fitting an aftermarket detent (search) may help if the part is not excessively worn. If you do not do this ,you will have the 2nd gear pop-out problem.That requires a gearbox rebuild to fix.
Check the engine mounts for leakage and check the front mount for cracks. Look for leaks from the w/p.
Be careful about selecting the most appropriate gbox oil.Lots of discussion about that here. Basically, the factory fill -Shell - is expensive but works !