Differential Fluid
06-03-2009, 10:48 PM
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Differential Fluid
I have a 97 A4 and need to change my fluid at 46,000 miles. I have read everything I can here on fluid recommendations and know that most of you like Redline, Amsoil, Royal Purple, and Mobil-1 synthetics. Some prefer OE GM with additive. My concern has more to do with the OEM fluid differences.
My 97 owners manual lists 12345977, an 80w90 petroleum based G-5 fluid. This was replaced by 89021669, which I believe is similar. These obviously need the addition of limited slip additive. I believe that in 1999, the recommended fluid was changed to 12378261, a synthetic 75w90 fluid. This was replaced by 89021677 which I believe is the grape scented synthetic. I don't know if this takes the additive or not? If anyone can add to or comment on these part numbers, please do so.
With this in mind, I read a discussion on a GM truck forum where an excerpt from a GM bulletin was posted that said limited slip differentials made prior to Feb of 1998 were susceptable to leakage if synthetic fluid was used due to breakdown of RTV and rubber seal components. They stated that vehicles made prior to Feb should use only petroleum based fluids with additive and should only change to synthetic fluid if clutch pack chatter was an issue. This would seem to coorelate with the change in 1999 Corvettes to synthetic. Do you think this could explain some of the leak issues that frequent this forum, at least on early models? Or is it simply the seal design? I'm still on the fence between using OEM (non-synthetic or synthetic) and aftermarket fluid.
My 97 owners manual lists 12345977, an 80w90 petroleum based G-5 fluid. This was replaced by 89021669, which I believe is similar. These obviously need the addition of limited slip additive. I believe that in 1999, the recommended fluid was changed to 12378261, a synthetic 75w90 fluid. This was replaced by 89021677 which I believe is the grape scented synthetic. I don't know if this takes the additive or not? If anyone can add to or comment on these part numbers, please do so.
With this in mind, I read a discussion on a GM truck forum where an excerpt from a GM bulletin was posted that said limited slip differentials made prior to Feb of 1998 were susceptable to leakage if synthetic fluid was used due to breakdown of RTV and rubber seal components. They stated that vehicles made prior to Feb should use only petroleum based fluids with additive and should only change to synthetic fluid if clutch pack chatter was an issue. This would seem to coorelate with the change in 1999 Corvettes to synthetic. Do you think this could explain some of the leak issues that frequent this forum, at least on early models? Or is it simply the seal design? I'm still on the fence between using OEM (non-synthetic or synthetic) and aftermarket fluid.
06-05-2009, 09:48 AM
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I don't think it is accurate to make blanket statements about "synthetics" and seal leakage. All synthetics are not equal. The API has not defined the term synthetic. In broad terms there are three groups of fluid that the NAD allows to be marketed as synthetic: Group III, highly hydroprocessed mineral oil; Group IV PAO or Polyalphaolefin, and Group V, all others which includes polyol esters. Each basestock reacts with porous materials differently (like seals).
Group III basestock fluids will probably interact with seals as would a mineral oil. PAO, such as Mobil 1 and AMSOIL use, tends to shrink seals. Polyol Esters, such as Red Line uses, tend to swell them.
Most synthetics contain higher levels of detergents (mostly in motor oil, not diff fluid), which will clean sludge from seals exposing them to oil.
Companies such as AMSOIL add a small percentage of Group V ester basestock to their oils to counteract the impact of seal shrinkage caused by the PAO basestock. The result is a fluid that will actually slightly swell seals (on the order of a few percent).
Group III basestock fluids will probably interact with seals as would a mineral oil. PAO, such as Mobil 1 and AMSOIL use, tends to shrink seals. Polyol Esters, such as Red Line uses, tend to swell them.
Most synthetics contain higher levels of detergents (mostly in motor oil, not diff fluid), which will clean sludge from seals exposing them to oil.
Companies such as AMSOIL add a small percentage of Group V ester basestock to their oils to counteract the impact of seal shrinkage caused by the PAO basestock. The result is a fluid that will actually slightly swell seals (on the order of a few percent).
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06-05-2009, 11:19 PM
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I don't think it is accurate to make blanket statements about "synthetics" and seal leakage. All synthetics are not equal. The API has not defined the term synthetic. In broad terms there are three groups of fluid that the NAD allows to be marketed as synthetic: Group III, highly hydroprocessed mineral oil; Group IV PAO or Polyalphaolefin, and Group V, all others which includes polyol esters. Each basestock reacts with porous materials differently (like seals).
Group III basestock fluids will probably interact with seals as would a mineral oil. PAO, such as Mobil 1 and AMSOIL use, tends to shrink seals. Polyol Esters, such as Red Line uses, tend to swell them.
Most synthetics contain higher levels of detergents (mostly in motor oil, not diff fluid), which will clean sludge from seals exposing them to oil.
Companies such as AMSOIL add a small percentage of Group V ester basestock to their oils to counteract the impact of seal shrinkage caused by the PAO basestock. The result is a fluid that will actually slightly swell seals (on the order of a few percent).
Group III basestock fluids will probably interact with seals as would a mineral oil. PAO, such as Mobil 1 and AMSOIL use, tends to shrink seals. Polyol Esters, such as Red Line uses, tend to swell them.
Most synthetics contain higher levels of detergents (mostly in motor oil, not diff fluid), which will clean sludge from seals exposing them to oil.
Companies such as AMSOIL add a small percentage of Group V ester basestock to their oils to counteract the impact of seal shrinkage caused by the PAO basestock. The result is a fluid that will actually slightly swell seals (on the order of a few percent).
