@mechmagcn
While I'm also doubtful that it would be your fluid and filter, it might however be a good (and cheap'ish) place to start.
On my '84 300D I lost 4th gear suddenly a few months ago. One minute it was there, and the next it wasn't. All the car would do is freewheel as if it was in neutral when it shifted into 4th, so 4th was totally gone.
I thought for sure my transmission was toast. Changed fluid and filter (and added 15 fl. oz of Trans-X fluid) and transmission has been working perfectly since.
Lost 4th gear....
Apparently fluid and tranny filters are insanely important on these transmissions, and the slightest impurity in the fluid or clogged filter will cause issues.