The older Paralever design (without the hole in the middle) employs a tough final drive design with a bearing arrangement that must be assembled with the correct preload on the bearing set. If your final drive has gone 50,000 miles without issue, then, it will likely go another 50,000 miles with only routine oil changes.
At about 800 miles, I dropped all the lubricants in my then new 2001 GS and went synthetic everywhere. The engine has had Mobil1 15w-50 ever since 800 miles and the transmission and FD have had 75W-140 synthetic GL-5 with added moly from the same mileage.
I change my gearbox oils at about every two years regardless of miles. I change the DOT4 in the brakes and clutch at the same time.
I first used BMW's 75w-140 but that stuff was red and waaaaay too costly. Now I used the Wal-Mart house brand Super Tech 75w-140. It has a good reputation and is priced at less that $10.00/quart.
My FD is OE tight and the transmission shifts like it's an automatic.
If you're concerned, flush the transmission and FD with ATF to help flush out any junk. Also, ATF is high in detergents and anti foaming agents so don't be concerned about any residual left behind. The idea is to get all the unwanted junk out of the gearcases during the flush.
Next question might be, change hot or cold? I prefer cold for changing all lubricants including the engine.
Final question might be, where do I get moly and what does it do?
Go here if you care.