One of the 2 local WMs close to me only had 2 quarts of the ST syn 75W-90 gear oil in stock for my 06 Tacoma rear differential, I bought them thinking I could grab another one from the other WM just down the road. Didn't work out for me, as the second store had zero in stock even though the website said they had 15. My brother has a full quart of Amsoil 75W-90 that I can use, my question is can I mix the 2 different gear oils without any problems?
Can I mix different gear oils?
- Thread starter hate2work
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dnewton3
Staff member
Yes.
I was under the impression that two GL-5 oils with different additive packages mixed appreciably (lets say >20%) does not provide very much a guarantee of still meeting GL-5 performance... Can more with industry knowledge weigh in on this?
dnewton3
Staff member
To the contrary, when something meets an API spec, one of the assurances is that another lube meeting that same spec will not adversely interact.
While the mixing may slightly degrade the performance capabilities of one relative to the other, there's not going to be any detrimental outcome which would harm the equipment.
In this case, the Amsoil is probably the Severe Gear; an incredibly capable lube with outstanding performance. The ST lubes are in no way bad, but they're not up to the level of SG. When mixed, all you'll get is some degradation of the SG capabilities, but nothing which would hurt the diff. I don't know if SG is API licensed, but I'm confident it won't induce any ill effects.
While the mixing may slightly degrade the performance capabilities of one relative to the other, there's not going to be any detrimental outcome which would harm the equipment.
In this case, the Amsoil is probably the Severe Gear; an incredibly capable lube with outstanding performance. The ST lubes are in no way bad, but they're not up to the level of SG. When mixed, all you'll get is some degradation of the SG capabilities, but nothing which would hurt the diff. I don't know if SG is API licensed, but I'm confident it won't induce any ill effects.
I've actually read (can't remember where but it wasn't just some forum where it was "I heard...") that you shouldn't mix them. I'd guess this really only comes into play if you're topping off the fluid though 'cause even if you do a replacement, there's still some fluid left in the gearbox or differential. I can't believe that API would allow incompatibility like that though.... What additive(s) used by one company will actually neutralize an additive used by someone else ?
I'd hope it simply knocks out the supposed benefits or added benefits while still maintaining or meeting the appropriate spec (GL-5, for example).
Like you, I've had no luck finding ST 75w90 or 80W90 in person, but ordering online and waiting a few days was successful.
The last service I did on my Chevy last year I mixed vavoline and supertech together and they have been enjoying life in the rear end via forced marriage. It doesnt get driven often but after about 1500 miles it hasn't exploded.
The last service I did on my Chevy last year I mixed vavoline and supertech together and they have been enjoying life in the rear end via forced marriage. It doesnt get driven often but after about 1500 miles it hasn't exploded.
I've mixed gear oils forever. Back when we actually serviced cars, 1972'ish, If you checked the rear end and it was low, you grabbed the 90wt overhead retractable hose from bulk oil barrel (remember them?) and added oil to fill rear end. This gear oil went in everything. Ford, GM, Mopar, Rambler etc. Sleep well.
Can yes but maybe shouldn't. If the gear oils meet the same specs (not only viscosity) I don't think harm will come from mixing them.
I just checked (online) at a nearby WM and it says they have it in stock. Their website shows two different ST 80W-90 conventional gear oils, one available, the other not. Could be a regional shortage.... How long have you been unable to locate it ?
I had 3 different brands of 75w90 in differential before and they all worked normal. Should be ok mixing them
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