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The MT-90 and the ....NS140 will likely be too heavy for your winters. 75W90 has the widest temperature range of all the ones you list. I'm personally fond of the 75W90NS because it works so well with high mileage transmissions. Works to take the grind out of 2nd gear better than any tranny oil I know.

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I believe MT-90 is also a 75w90 oil (and MTL is thinner,) but MTL and MT-90 are both GL-4 lubricants and are not recommended for Subarus because the hypoid front differential is part of the tranny and shares the same lubricant. Apparently the hypoid gears need the differently formulated GL-5.

 

That leaves 75w90 and 75w140. I don't think one would ever want to use the heavier oil in our cars in any environment but definately not a cold one. When you first got rolling in the mourning each shift would take minutes!

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I've run Mobil 1 75w-90 in my tranny and rear diff in a couple different cars and have been very pleased. I've spent this winter in anchorage and it's always shifted well. Did fine driving through Tok at -45 a month ago too.

 

Theoretically the 75w-140 is going to flow just as well at low temps, but I doubt it does because it's stretched over such a huge range. In a subaru, you'd never see any benefit from running that over the 75w-90 anyway, especially up here.

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I just put RedLine 75W90NS in my '02 OB back in Aug or so. So far I'm pretty happy with it. Shifts are smooth and it's cold weather viscosity is good (only got down to ~0F or so).

 

I had previously used Mobile1 75w90, it wasn't quite as smooth. Biggest problem I had was it seemed to thicken up a lot when it got very cold. One trip to VT at -25F and shifting was very difficult until it warmed up.

 

Some others on this board recommend Valvoline Synpower, I think there are a few threads out there.

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So, if I wanna go with redline I'm going for the 75W90NS correct?
Depends on the age of your transmission. Redline makes both a 75W90 and 75W90NS. Either will work well for you, but the NS version is recommended for older gearboxes since they have an additive that allows old slippery synchros to grab better and reduce that nasty crunch when quickly shifting into 2nd (or other gears).

 

When I bought my '91 Legacy 8 years ago it had 93K on it and a slight crunch going into 2nd. I think the seller thought the transmission was going out so he wanted to unload the car. I changed the transmission oil to the 75W90NS and the crunch went away after about 3 weeks of driving.

 

When I had about 200K on the car I changed the tranny oil again (ooops, a little late don't you think:rolleyes:?) but didn't have any of the Redline at the local autoparts place so I bought the same grade of Lucas gear oil. The evil crunch was back big-time going into 2nd, 3rd and even 4th!:mad:

 

After a week of that, I drained out the Lucas gearoil, and replaced it with 75W90NS. Transmission shifted smoothly again in all gears.:) It made a believer out of me.

 

Car now has 217K+ on it and it's still going strong.

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Theoretically the 75w-140 is going to flow just as well at low temps, but I doubt it does because it's stretched over such a huge range. In a subaru, you'd never see any benefit from running that over the 75w-90 anyway, especially up here.

I used the 75W140 once when I was trying to reduce the stress on the Impreza's transmission due to high rpm's during rallyX events. I took it out after a week because it felt like the shifter was mired in butter (and rallyX is normally a summer sport).
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