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General Articles - Product Reviews

Red Line Oil MT90

Article written by Moab Man

Date Added: 01/16/2004

Changed your manual transmission gear lube? Have brass synchros? Learn about this top notch product and the damage that could be happening to your transmission without it.

If your Jeep has the AX-5 or 15 manual transmission, and you have changed the gear lube, you may be slowly damaging your transmission. Both of these transmissions, and others, have brass synchros. This type of transmission needs a GL-3 or 4 gear lube. After a trip to your local automotive store you will find that GL-3/4 fluids are no longer available. At least we couldn't find any. In its place you will find GL-5 marked as covering all applications GL-3 through 5.

GL-3-5 all purpose lube will work for some transmissions, but GL-5 lubes (with a few exceptions) contain sulfur. Brass, being a soft metal, is dissolved and eaten away by sulfur.

So what are you to do?

This is where some of your high end synthetic oils; Amsoil, Lucas, Royal Purple, and Red Line come into play. All of these companies offer gear lubes covering your transmissions needs without the damaging sulfur.

Why Red Line if all these oils cover my needs without the sulfur?

Within your transmission is a friction dichotomy. Your Jeeps transmission needs as much slickness as possible to prevent wear, but at the same time you need a certain amount of friction so that your transmission operates properly. A transmission using GL-5 is going to have a different slick/friction need than a GL-3/4 transmission. And many GL-5 transmissions use sulfur in the lube to keep internals clean and gunk free. This is where the MT90 fits our AX-5, AX-15, or any other brass synchro transmission needs... it has NO SULFUR.

So how might you be damaging your transmission?

Many lube shops and dealerships may be unaware of the damaging effects of an all purpose lube covering GL-3 through GL-5 (sulfur containing) lube. I found this out when my local Jeep dealership recommended a Mobil 1 synthetic lube. Per their recommendation, I put it in my transmission. Over the course of a couple weeks my transmission started acting up. No longer did it shift very well, and I started catching gears when shifting. The dealership didn't have any suggestions or solutions as to what happened and why I'm having problems out of the blue.

Finding no answers from the Jeep dealership "experts", I started doing research in other places. This was when I first learned GL-5 typically contains sulfur, and of the consequential brass issue. I mentioned it to the dealership and they were unaware of there being any compatibility problems.

My transmission was getting noticeably worse, and I wasn't getting any further in figuring out the answer, so I contacted Mobil Oil directly. I told them what transmission I had, and the course of events leading up to my transmissions health turning south. They immediately responded with a direct order to get this lube out of my transmission. They explained that it had a HIGH content of sulfur for GL-5 applications (although the product covers GL-3-5) and should never be used in a brass synchro transmissions.

Final Thoughts

As I have said earlier, any of these high end lubes will meet your needs, but why not use a product such as MT90 that is tailor made for our brass synchro needs. Our personal experience with this product has been outstanding. Specifically, smoother shifting with an immediately noticeable reduction in gear grinding. MT90 could never fix the damage done, but it certainly has improved the performance in spite of what the sulfur chewed on.

As for Mobil Oil, thank you. What happened to this transmission was in no way their fault. They make a high quality product, and like any product, it must be used in the right application.

Our Thank You's!

Thank you for reading MOABJEEPER Magazine.

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