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I'm looking at replacing the gear oil in the axles of my '97 Legacy wagon (108K) with synthetic. South Dakota gets cold in the winter and hot in the summer. Any potential problems like leaking seals, etc?

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Replaced both front and rear differentials with Mobil-1 75w-90 at 100K. Now have 130K and no problems. Winter (Minnesota) shifts seem a bit smoother with the synthetic gear oil - and I'm still running non-synth tranny fluid.

 

96 OBW/auto 2.5

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I use Mobil 1 75w-90 as well in the diffs and have been very pleased with it. I am using valvoline sythetic blend ATF in the tranny. I still change both every 30k.

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Search some of my posts, and you will find that I am a die-hard dino oil fan. I have had nothing but bad experiences with synth gear oil in my Legacy...

 

However, you up in the colder regions of the country have nothing but praise for it...

 

So here is my theory on dino vs. synth: Synth is okay in colder climates, where the thickness of cold oil can make up for the slipperiness of the synth stuff. But here in hot weather, the oil gets thinner and the same slipperiness does not allow proper gear synchronization....

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I know I've said this before, but switching to Quaker State synthetic 75w-90 in my subaru was one of the best choices I made relating to my car, and definitely worth the cost. Cold weather or hot, not a single drip from the tranny or the rear end (just the engine:rolleyes:), and she shifts SOOO much smoother now. It's also pretty easy to shift right when she starts up in below zero weather, where before with dino oil, it was difficult to shift for the first few miles in temperatures below freezing.

 

And I did the switch on a tranny with 150,000 on it, which is WAY after you're supposed to do it.

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I know I saw this mentioned in a seperate thread, but I'm not all that clear..

 

If you use a synth gear oil in a 5MT, should you, or do you need to add a friction modifier? Same goes if you have the VLSD, since the lsd isn't mechanical, should you use a friciton modifier anyway, or are you better off without it? :-\

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Danish winters can get pretty cold and I have found a BIG advantage using synthetic.

 

Redline is a notch better than Mobil, but both beat all the competitors hands down.

 

Redline 75W-90NS in the manual 'box, shifts very smoothly in hot and cold weather - M1 lags little in extreme cold. Both have the friction modifiers in them already. Redline make a 75W-90 for non-LSD diffs, which would be good to use in conjuction with an autobox.

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Thanks for the info, guess I should have read the mobile 1 bottle a little more closely. It doesn't get all that cold here, but I figure I'll give it a try the next go around.

 

Thanks.

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I've used the Mobil 1 75w30 in the rear differential for over two years with no problems. The power transfer is smoother and I get less binding in turns when the tires are out of measure with each other.

 

I use valvoline synthetic in the front diff because subaru suggests something a bit thicker than 75w 90 in the front differential. I forget the weight, 85w 90 or something similar.

 

I use regular ATF fluid because it is cheap and I hate my transmission.

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