I recently changed the gear oil in my manual tranny. I used Mobil 1 synthetic 75w90 gear oil (or whatever the Volvo manual's recommended weight was). It was an immediately noticeable improvement, and my car only has 78,000 miles on it. The M1 is around $10 a quart. If I were to do it over, and I just might--I would go with Royal Purple.
Get 3 quarts of whatever gear oil you choose, as you will need just over two quarts to refill. Also, the oil will flow forward over the chassis beam when you open the drain plug and form a Niagra Falls effect so be ready with a large open container. The fill plug is a little hard to access. Make sure you have a deep, thin socket or you will be left high and dry.
I would not use motor oil in your transmission, as some suggest above.
Do it. It's a cheap but noticeable improvement. You will love the difference.
It's funny that VADIS doesn't actually state what the Volvo gear oil is beyond saying that's it's synthetic and the equivalent of an SAE 5w 30 oil.
The AWD angle gear oil is an API-GL5/SAE 80W synthetic gear oil which is reverse compatible with the M56/M58/M59 transmissions according to the bottle so I assume that the transmission gearbox oil is a similar weight or Volvo wouldn't say that the 2 gear oils are reverse compatible.
This is why I stated that it's a 70 80 gear oil...I am going to have to look carefully at a bottle of the stuff soon and report back.
Nothing wrong with motor oil. I use it in my Saab manual. My old 740 shifted better after going back to 5W-30 from MTL. I love and use Redline motor oil, but the MTL is not friendly to every MT, and was basically too slippery for the sync's.
These days, I personally would use the OEM stuff.
I found the shifting effort to be higher when engaging any gear. This shocked me, since I thought I had changed to the absolute best stuff there was. Then I started researching this and found that this was a common complaint about MTL on the Honda forums. They have better luck in their cars with Honda's product. I went back to motor oil, and things were much improved.
Ironically, Redline's own product info says the MTL lubes "have the appropriate coefficient of friction for most manual transmission synchronizers. Many gear oils, engine oils, and ATFs are too slippery for proper synchro engagement."
I found the effect to be the opposite. Of course, this was on an older tranny with 150K on it. YMMV.