My dear wife drives a 99 S70 T5 5 speed stick shift (original clutch) that has just clicked 300k miles.

The trans oil has not been changed in a long time.

Which/What Oil should i put in there?

Other sites tell me stick with OEM stuff $$$$$.

Some say Mobil1 5/30 motor oil.

 

Volvo specifies the use of "Volvo Synthetic manual transmission lubricant." In a later technical bulletin, they elaborate on this, saying you can use "API GL-4, Synthetic." GL-4 is a gear lube specification. For some dumb reason, they don't list a weight. Some people run Mobil1 5W30 to reduce the amount of energy needed to move the fluid, as normal gear oil is very thick. I would NOT do this! The current Volvo stuff is "75W." They don't provide a warm temp viscosity, only the cold temp viscosity, the "75W." I would use synthetic 75W90. I'm not sure where other folks get the idea that the Volvo stuff is just synthetic 5W30 oil.

On the other hand, I've seen people all over different forums who seem to be running all kinds of different things in there- ATF, 5W30, conventional gear lube, Royal purple MTL, and it appears that these transmissions can handle anything and keep on going.

This leads me to one of my pet peeves. OEMs who REFUSE to release specifications on their vehicle fluids, when they know full well that it's some sort of off- the- shelf fluid that could be obtained elsewhere for 1/4 the price IF you only knew what it was. VW is the king of this crap- they have every VW owner convinced that allowing a single drop of any sort of other coolant, other than VW's $20/1.5L coolant, into the engine will cause total destruction of their engine, which is nonsense. They also have a $20/L ATF that you have to use in their cars, though that can be obtained now elsewhere for around $16/L. Lately, Volvo's been doing the same thing, though they do say you can use "equivalents," without specifying what those might be.

 

Thank You Mr Carrots,i can see where people get their idea's.

Consider that ZF manual transmissions in F series Ford Trucks use regular old ATF.

Some time ago i owned a Porche that used a real thin hydraulic oil in the transaxle.

I can buy the Volvo approved stuff for not much more than a branded synthetic oil. I may just go on that. Thanks again for your input.