Gear Oils 70w80 vs 75w90

[Ranton]

So I'm bored at work and been reading about Gear Oils again.. (Its a Quiet Day So sue Me :P)

Now I read somewhere that the Gen II MPS now has factory oil reccomendation of 70w80 as opposed to 75w90

This is supposed to improve the cold weather performance of the gearbox and make it smoother.. Wether or not this is at all valid I haven't a Clue. But I thought a thread for purely informational purposes would be good for the Wider community.

Is there any validity to what I have read above regarding the difference in cold weather performance.. or just over all smoothness of the gear oil...

Or is this all just another Wives tale/what have you?!

[rd415]

Showing my age again - in the late 60's we used to put ATF (Auto trans fluid) in the Aus GMH 3 speed box and the 4 speeds instead of EP90 to ease the blocking out of second gear when cold. Some new boxes were so bad you had to skip second until the oil warmed up. Worked well, but to be on the safe side 20w50 engine oil was commonly used.

The thinner oil will make it easier to change, but it's not much difference in viscosity so it wouldn't make a huge difference.

[Ranton]

Engine oil in a Trans would be interesting.. how would the synchros operate?

[kagerou]

how cold does it get where you are now? and are you having trouble changing into reverse or second when you first start up in the moving?

how hard do you drive the car or the gearbox? and what are the ambient temperatures that you most commonly drive in?

you have to consider all these variables to see whether you need to go higher or lower viscosity oil.

the thinner oil will make it easier to shift on a cold start, but might not be suitable in conditions where the oil could get quite hot, eg. 40 degree days or long trips on the highway.

if you were to go lower, then i would pick 75w-85 rather than 70w-80, that is unless it is consistently very cold in winter.

[Ranton]

for Example The hottest We see here at very best is 32 deg

one day a year.. if we are lucky

the Average Day in summer is around 20 deg

the rest of the year is between 4 and 17 deg in the main city and even lower further down the country..

Say I plan to be heading to the Snow Quite often this winter would i be better running the thinner viscosity oils especially as the 1st and 2nd Gears are the hardest to move first thing in the morning.. after the initial heat up they arent so bad..?

[rd415]

Engine oil in a Trans would be interesting.. how would the synchros operate?

No problem - in fact the viscosity of gear oils is tested differently to engine oils, in viscosity tests eg a students exercise - heat oil to a set temp and measure the time (with a stop watch) a set amount takes to pour through a small tube on the end of a funnel. Quite startling results in the closeness of time (viscosity) at relative high temperature.
A lot of the newer large farm tractors are now using engine oil in their diffs. Mitsubishi Sigma borg warner 5 speeds use auto trans in their manuals. Ford have been using auto trans fluid in their manuals as well as engine oils since the XD Falcon. It is not uncommon to use engine oil in gearboxes.

Just pick up any service station lubrication guide and have a look