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Old 08-03-2005, 02:25 PM   #1
eta_Legacy
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1999 Leg. L 30th Ann

Default Differential oil, synthetic?

I've been running Mobil 1 Synthetic ATF in my Legacy 2.2l auto, and it seems to have held up well.

I'll be performing a lot of maintenance on the car soon, and would probably like to change the front and rear differential oil. The last time I had this done, I watched the guy drain the diffs and then pump in thick mineral gear oil.

This time, I'm interested in going to Mobil 1 gear oil in the diffs, so I have the following questions:

1. The product they list on their web page is "Fully Synthetic 75W-90 gear lube" http://www.mobil1.com/USA-English/Mo..._75W-90LS.aspx but it does include LS friction modifiers. Will this be a problem? I believe my model Legacy has open diffs in both front and rear.

2. Is anyone else here running Mobil 1 in their diffs, or any other synthetics? If so, does the oil you're running have friction modifiers?

Thanks.
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Old 08-03-2005, 03:38 PM   #2
gtwagon941
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I'm running Redline synthetic on mine. No issues for me. Not sure about the mods though.

Jared
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Old 08-03-2005, 03:54 PM   #3
xcntrk75
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The Legacy diffs are open (non LSD) and do not require any additive. In-fact, even the LSD units do not require additive as the slip mechanism is a sealed assembly, any strait 75wx90w will be fine.

I used to run Mobile-1 and hated it! There are a number of owners here who also dislike it and would agree with me, and an equal number who probably would swear by it.

Mobile-1 (motor) oil has a tendency to do the disappearing act on “turbo” powered cars. Mobile-1’s 5w-30w motor oil (our mfg’s recommended weight) is so fricking thin it seems to get burned/used in the turbo internals and intake blow-by. I went through 1 oil change with Mobile-1 and was constantly topping off the fluid level over the course of 3k miles. I’ve since changed to Royal Purple and have been much more satisfied. Again, this is in a turbo motor.

You hear lots of complaints with Mobile-1 gear oils being too light as well which allows more room for grinding. I too run Mobile-1 in my MT, front and rear diffs. I’ve experienced slight grinding on occasion going into certain gears since introducing this type of gear-oil. That said, this too is getting changed out to Royal Purple simply because I hate the motor oil so much...
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Old 08-04-2005, 01:37 AM   #4
eta_Legacy
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Quote:
Originally Posted by xcntrk75
The Legacy diffs are open (non LSD) and do not require any additive. In-fact, even the LSD units do not require additive as the slip mechanism is a sealed assembly, any strait 75wx90w will be fine.
Right, I was just wondering if the friction modifiers that are already in the commonly available Mobil 1 gear oil will cause a problem with the open diffs.

Quote:
Originally Posted by xcntrk75
I used to run Mobile-1 and hated it! There are a number of owners here who also dislike it and would agree with me, and an equal number who probably would swear by it.

Mobile-1 (motor) oil has a tendency to do the disappearing act on "turbo" powered cars. Mobile-1's 5w-30w motor oil (our mfg's recommended weight) is so fricking thin it seems to get burned/used in the turbo internals and intake blow-by. I went through 1 oil change with Mobile-1 and was constantly topping off the fluid level over the course of 3k miles. I've since changed to Royal Purple and have been much more satisfied. Again, this is in a turbo motor.
My experience has chiefly been on NA cars, mostly the typical daily driver. I've been using Mobil 1 motor oil since about 1991, had great success with it on personal cars, and have recommended it to friends and family who have cars they care about. However, I've no real experience on turbo platforms, so I fully defer to your knowledge here. If/when I go turbo, I'll probably look very closely at Amsoil, Redline, Motul, and the advanced/foreign forumlations of Castrol Syntec (fully synthetic). Given I'm not the first owner of my car though, Mobil 1 is probably the best compromise of protection and expense/availability for me.

Quote:
Originally Posted by xcntrk75
You hear lots of complaints with Mobile-1 gear oils being too light as well which allows more room for grinding. I too run Mobile-1 in my MT, front and rear diffs. I've experienced slight grinding on occasion going into certain gears since introducing this type of gear-oil. That said, this too is getting changed out to Royal Purple simply because I hate the motor oil so much...
I've heard a lot about transmission problems when running Mobil 1 gear oil in manual transmissions, on Subarus and other makes as well. It seems that for each different brand/fluid, people report problems, especially with the Subaru manual transmissions, which seem to be particularly finicky. Last I'd heard, "Uncle Scotty's home brew" manual transmission mixture was working well for the people who'd tried it.

However, all that's neither here nor there for me since I'm driving a slushbox.
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Old 08-04-2005, 02:20 AM   #5
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cobb used to have an extensive write-up on the pros and cons of different oils and why to use them and when to use them. I don't know if that page is still around though... i read this back when they were still ocated in texas...
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Old 08-04-2005, 11:56 AM   #6
eta_Legacy
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Quote:
Originally Posted by gtwagon941
I'm running Redline synthetic on mine. No issues for me. Not sure about the mods though.
Are you running the the 75W90 NS or the "regular" 75W90 ?
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Old 08-04-2005, 02:46 PM   #7
plasticguy
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my general experience with synthetics has been to love it in the engine (non-turbo, particularly in winter when starting is made easier), love it in the MT gearbox, and hate it in differentials.
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Old 08-04-2005, 03:50 PM   #8
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I had a problem with low diff fluid in my front diff (4EAT slushamatic) recently. I purchased some Mobil 1 75-90w synthetic, and the problem had pretty much gone away, except on very hard locks at speed. I only topped up, so I need to drain it properly this weekend.

I am currently using Mobil 1 synthetic in the gearbox too, I had quite a hard slam 1st-2nd change, and that dimished a lot for about a week after I drained and refilled 6qts.

When I next change out, I'm going to try using a different gearbox oil, to see if there is a better result.

Diffs & gearbox have 174,000 miles on them.
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Old 08-04-2005, 05:33 PM   #9
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I've been using Redline 75w90 in the rear diff, and 75w90NS (no frict mods) in the Tranny. Search the maint. forum... this subject has been discussed extensively.
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Old 08-04-2005, 06:29 PM   #10
emyselfandi
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I run mobile1 syn. gear oil in my jeeps diffs. I normally go with the reg. stuff but it was only 50 cents more so i figured what the heck.. I don't think it makes that much of a difference. The biggest part is changing the fluid like clock work when it needs to be done. I run reg. ATF in the auto and I think i've read that syn. ATF isn't good for the 2.2's auto tranny???? I did switch to syn. in the 2.2 and it has helped with the valve tick and start up.

When you do the fluids go to the stealership and get new crush washers. Just get like 10 of them and your set for a long ass time. they are like a buck a piece. if you don't the bolts will leak. The front diff. will leak onto your cat and smell like crap. don't ask how i know this....

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Old 08-05-2005, 03:29 AM   #11
eta_Legacy
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Quote:
Originally Posted by snowone
I've been using Redline 75w90 in the rear diff, and 75w90NS (no frict mods) in the Tranny. Search the maint. forum... this subject has been discussed extensively.
I did search, but:
1. Most posts had to do with the WRX, which has at least one limited-slip diff, and
2. Most posts also dealt with the gear oil pertaining to the transmission, mostly the manual.

Since I'm talking about Legacy open diffs, and specifically not the transmission, I didn't see any applicable threads out of the ton of results that were returned.

If you can find a particularly applicable thread by searching, it could just be that I suck at searching, and I'd appreciate any good info you could find.
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Old 08-05-2005, 02:21 PM   #12
snowone
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THis is the one if you have time to read: http://forums.nasioc.com/forums/showthread.php?t=416514

FWIW almost all new/current info is going lean toward WRX/STi. I suggested searching, as I was in the same boat years ago on this subject, READ A LOT OF INFO HERE, and have posted what I ended up using/doing.

.02 is use Redline 75w90 in your rear diff.
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Old 08-06-2005, 12:12 AM   #13
janikphoto
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you can do what I did and fill your rear diff with crisco. just be sure to use the the animal fat, better known as lard, because vegetable shortning just won't cut it!!!
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