Gear Oil Versus Motor Oil for a Manual Transmission


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Posted · Report post

Hi guys. I have an odd question, and since i keep getting different answers, i figured i would ask here for some clarification. My question actually pertains specifically to the AX15 in the Dakota, but maybe this can be used to start a discussion about other transmissions as well. My question is simple, which is better to use in the Dakotas AX15, regular 80w-90, Pennzoil syncromesh, Lucas heavy duty 80w-90, or 50w engine oil? My truck sees extreme cold and warm weather, limited 4 wheel drive use, mild to once in awhile heavy towing, and frequent short trips and large amounts of highway miles. My idea was to use one quart of Lucas heavy duty 80w-90, and one quart of regular 80w-90, but some people say not to do that, and others say dont use the Lucas at all, so which is the right answer?

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Posted · Report post

Also, does anyone know exactly how many quarts that transmission takes? I dont have the owners manual for it, and i have heard anywhere from 2.5 to 3.25 quarts.

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Posted · Report post

Hi, Biggman100...I ran this down with research.  Mopar had a specific lube for the AX5 and AX15 transmission that is no longer available.  I used it in the AX-15 rebuild at the magazine, so the first fill was a non-issue.  It got dicey after that when many followed a Mopar superseded approach with a 10W-30 motor oil (presumably synthetic?) replacing the discontinued Mopar AX5/AX15 lube.  (I may still have a few bottles of that lube left, likely an eBay NOS collectible item at this point.) 

 

After watching this question kick around in the forums and elsewhere, I chased down the Toyota A150 transmission approach.  The A150 is the AX15.  Toyota recommends an API 75W-90-wt. gear lube in GL4 or GL5 class*.  I'm good with this idea, although there may be a synthetic alternative from Redline or Amsoil worth testing.  Each makes direct replacement synthetic lubes for GL4 or GL5 gear lube.  Be aware that synchronizers are very touchy on these transmissions, and too much lubricity is not "better".  I found the Mopar lube (original) worked well, the synchronizers maintained braking effect.

 

*Note: Unlike a lot of speculative approaches to this question, my information is from page MA-17 of the 1993 Toyota T100 factory shop manual, Publication RM299U for North American market.  I thought this as close to your AX15 in the Dakota, Biggman100.  My shelf is lined with Toyota truck manuals from the AX15 era, 1989 up in the case of Jeep and Dodge Dakota.

 

What I would not do is run a motor oil, and for one simple reason: Motor oil is not an EP rated lubricant nor intended to be.  It does not have the same degree of anti-foaming or "squeeze factor" engineering as a true gear lube.  

 

If there is an issue with a conventional 75W-90 weight lube, I'd consider an alternative.  Otherwise, I would sleep nights with the Toyota recommendation.  When I recommended synthetic gear lube for any manual transmission back in the 'eighties in a magazine Q&A response, a Toyota 4x4 truck reader wrote me kindly and suggested that his gears were gnashing after the change.  He switched back to conventional gear lube, and the problem went away...That was the end of my making arbitrary statements about the virtues of synthetic lube.  Synchronizer braking action requires a prescribed degree of friction. 

 

Others may have more to add or suggest...

 

Moses

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Posted · Report post

Moses, as always, thank you for that information. One other question though, would you recommend a regular or synthetic gear oil? I found both a regular GL-5 75w-90, as well as synthetic 75w-90 from Lucas, Mobile, and Valvoline. I am leaning more towards the Lucas synthetic, because i have had good results with Lucas products in Dakota automatics, but i have never tried it in a manual transmission.

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Posted · Report post

Biggman100...We're back to the synchronizer response.  If you can get proper synchro braking (smooth shifting without clashing) with the synthetic, use it.  From my recollection, the original Mopar oil was not a synthetic, possibly a blend, it looked and poured like a conventional 75W-90 oil. 

 

Toyota does not specify a synthetic, either.  I am a synthetic buff, too, so if you can use a synthetic and still get proper synchro response, great.  An expensive experiment if the gears clash with synthetic and you need to change out the oil. 

 

Moses

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