This is what i've gathered from my own knowledge. hope it helps.
The 5 speed transmissions in the EVO 4-MR tend to shift best with a GL-4 gear oil that meets specification for synchromesh. Like several evo forummers have stated (evo forums), do not use a GL-5 in the transmission. I would avoid using any gear oil that claims to be GL-3, 4 & 5 spec. Instead, option only for a GL-4 oil that meets.....specification for synchromesh. These oils will improve the effectiveness of the synchronizers and that means a better shifting transmission.
GL-5 oils should not be used in the Mitsubishi transmission because they are too slippery and reduce the effectiveness of the synchronizer and you don't want that because the synchros basically act like brakes, their job is to bring the speed of the various transmission components up to the same “synchronized” speed with one another so the engagement teeth mesh at the same speed and don't get all torn up.
When it comes to the 6 speed Mitsubishi specifies the Diaqueen Super Multi Gear Oil, part #MZ313376. Could you use a different oil in this transmission? Most likely you could but consider this, Mitsubishi rarely uses the phase "exact equivalent" and when they state to use the Diaqueen SMGO or the "exact equivalent" it's probably best to follow their advice.
In the transfer case and rear differential you must use a GL-5 "Hypoid" type gear oil. The transfer case features hypoid gears to transmit that power to the drive shaft. The word hypoid is short for hyperboloid which refers to the design of these gears. All you need to know is that hypoid gears; don’t share a common axis, run quiet (for the most part) and they are a strong design. Yet by design the gear teeth must slide across one another during their meshing and this is why you must us a GL-5 Hypoid type gear oil in the transfer case and rear end, you need that slippery oil here. Use the wrong oil and the transfer case will start howling and growling within a couple tanks of gas. Mitsubishi specifies the Diaqueen LSD Gear Oil in the transfer case and rear end of the EVOs. This oil is a single viscosity, 90 weight, and the part number is #377610. If you can’t find this oil you must find a gear oil that meets the GL-5 Hypoid specification. Again this oil must ONLY meet GL-5 preferably 90 weight. Do not use a gear oil that claims to be GL-3, 4 & 5. These one size fits all type oils are not recommended anymore than a one heat range fits all spark plug.
Also, multi viscosity index modifiers and dyes both reduce the film strength of the oil; they take up valuable parking spaces on the molecular level and that’s what the transfer case and rear end both need more of, film strength. There are several brands of oil on the market and you can use any of them as long as it meets Mitsubishi's requirements.
Below, the requirements for oil/lubricant replacements.
1)
Item :-Transmission Oil
Specs :- Gear oil SAE 75W-90 or 75W-85W conforming to API GL-4
Quantity :- 2.8ltr
2)
Item :- Transfer oil
Specs :- Mitsubishi Genuine Gear Oil Part No. 8149630 EX, CASTROL
HYPOY LS (GL-5, SAE 90), SHELL-LSD (GL-5, SAE 90W-90) or
equivalent
Quantity :- 0.55ltr (RS), 0.6ltr (GSR)
3)
Item :- Fluid
Specs :- DIA QUEEN ATF SP III
Quantity :- 0.9ltr,, 1.0ltr (wuth AYC)
4)
Item :- Gear Oil (vehicle with mechanical LSD)
Specs: :- Hypoid Gear Oil
Mitsubishi Genuine Gear Oil Part No. 8149630 EX, CASTROL
HYPOY LS (GL-5, SAE 90), SHELL-LSD (GL-5, SAE 90W-90) or
Equivalent
Quantity :- 0.55ltr
5)
Item :- Gear Oil (vehicle with ACD/AYC)
Specs :- Differentials – Hypoid Gear Oil API Classification GL-5 or higher SAE
viscosity No.90W,80W
Torque Transfer Mechanism – ATF-SP
Quantity :- 0.55ltr
0.65-0.7ltr