Gear lube is not just about viscosity. Lube base stock is critical in heavily loaded transmission. Reason Mercedes spec'd a synthetic base stock lube for the G56 in MD real truck apps. read the whole Mercedes spec, it's in German but easily translated.
Vis @ 40°C and Brookstone are indicative of cold startup viscosity
Vis @ 100°C, cSt is indicative of running at spec temp conditions viscosity.
Higher viscosity lubes provide more film protection between the meshed gears and between bearing components.
It's a tradeoff between shift quality from lower viscosity and transmission protection/long life.
Brookfield Viscosity @ -40°C, Poise is another indicator of lube performance at low temps. Lower the #, less the viscosity and gear/syncro drag.
Lower viscosity, especially at cruising temps, reduces gear/bearing drag thus increasing fuel mileage mpg, money in your pocket.
The ester real synthetic base stock in Redline lubes has another advantage, adhesion film to metal bonding.
General Maintenance - Crankcase Oils
Esters have all the advantages of a PAO but more of them. Esters can handle heat better than PAO’s and when burned, esters leave far less coking deposits. Esters are static types of oils and are attracted to metal parts with an electro-chemical bond. This means no more metal to metal start ups. This also means that a film is there before the oil pressure light goes out preventing premature wear of high-stressed parts like cam lobes. The film created is up to 5 times stronger then petroleum oil.
The number one reason to run an ester synthetic oil is bond. The electro-chemical bond is made because the ester molecule is polar. Sort of like a refrigerator magnet. It is attracted to metal and sticks.The PAO molecules are neutral and act like a piece of plastic placed on the fridge. They just fall off. All commercial jet plane flying, use an ester synthetic of some type and not a PAO. You need to run an ester of some sort for maximum protection.
Friction modifiers, antiwear additives and extreme pressure additives types and amounts used in the additive package to the base lube all affect how syncros function, cold and hot. They are not the same between brands of lube oil.
MTL for cold climates and/or NO>Lt. loads
Vis @ 100°C, cSt 10.6
Vis @ 40°C, cSt 56.2
Viscosity Index 183
Pour Point, °C -50
Pour Point, °F -58
Red Line Synthetic Oil - Gear Oil for Manual Transmissions - MTL 70W80 GL-4 Gear Oil
MT-85 75W85 GL-4 Gear Oil for med. hvy loads and hot climates
Red Line Synthetic Oil - Gear Oil for Manual Transmissions - MT-85 75W85 GL-4 Gear Oil
Vis @ 100°C 12.0
Vis @ 40°C 64.3
Viscosity Index 172
Pour Point, °C -48
Pour Point, °F -54
I would not use the MT-90 75W90 GL-4 Gear Oil
except in the hottest climates towing max. loads. too much gear drag from the high viscosity.
SAE Viscosity Grade (Gear Oil) 75W90
Vis @ 100°C, cSt 15.6
Vis @ 40°C, cSt 90
Viscosity Index 185
Pour Point, °C -45
Pour Point, °F -49
Red Line Synthetic Oil - Gear Oil for Manual Transmissions - MT-90 75W90 GL-4 Gear Oil
Mobil Delvac Synthetic Transmission Fluid 50
I'd only use to reduce gear/shaft chatter if objectionable after conversion to a non DM clutch.
SAE Grade 50
Viscosity, ASTM D 445
cSt @ 40ºC 132
cSt @ 100ºC 17.5
Viscosity Index, ASTM D 2270 146
Pour Point, ºC, ASTM D 97 -45
Putting the Simple Back into Viscosity - Bob is the Oil Guy