What is your favorite manual transmission gear oil?

[Odoyle]

I want to restart the conversation over which type of manual transmission gear oil to use in our spiders. The running consensus in the forum archives suggests that GL-1 gear oil is the safe and correct choice for gear oil in our manual transmissions. But what is GL-1 exactly? All the factory manual states is the use of 1.75 qts. (1.65L) of "SAE 80W/90 oil (not EP) containing special anti-wear additives" is to be used.

"Gearbox oils are classified by the American Petroleum Institute using GL ratings. The higher an oil's GL-rating, the more pressure can be sustained without any metal-to-metal contact taking place between transmission components. Separate differential usually have higher pressure between metal parts than gearboxes and therefore need higher GL-rating. For example, most modern gearboxes require a GL-4 oil, and separate differentials (where fitted) require a GL-5 oil." (Wikipeadia)

"API Category GL-1 (inactive) designates the type of service characteristic of manual transmissions operating under such mild conditions of low unit pressures and minimum sliding velocities, that untreated oil may be used satisfactorily. Oxidation and rust inhibitors, defoamers and pour depressants may be used to improve the characteristics of lubricants intended for this service. Friction modifiers and extreme pressure additives shall not be used. " (Wikipedia)

In my experiences with GL-1 thus far, specifically SAE 90W mineral gear oil from NAPA, its performance does not seem adequate. The oil seems to meet the minimum requirements for our cars, but still does not contain any of the "special anti-wear additives" fiat recommended. And it seems almost right to chose such a "vanilla" option of gear oil considering all the warnings about how EP additives contain phosphorus/sulfur compounds which are corrosive to yellow metals such as the copper and/or brass used in bushings and synchronizers. BUT surely there has to be better option than plain 90W mineral oil with zero anti-wear additives. So I encourage all of you to join the conversation and share your experiences with GL-1 gear oil or any type of gear oil you are using or have used in your spider.

I'll start first by saying that in the name of science, I have been trying out a new gear oil called PennGrade multipurpose Classic GL-4 gear oil which is; "a specially blend using high-quality PennGrade mineral base stocks and select additive technologies. It is designed to be used with yellow metallurgy (brass, bronze, copper) and other soft metals used in synchronizers, bushings, thrust washers, and other components typically found in classic manual transmissions and transaxles." The online reviews are all very positive, the MG and Triumph guys have good things to say about it and I even read that it was working well in some Vespas. And
after about 500 miles with the near gear fluid, I am very happy with the feel of the shifts at all temperatures, and the gearbox sounds a little more quiet.

[tima01864]

I have used this on my older 124"s, Not recently but an alternative to NAPA GL1
https://www.sportscarcentre.ca/transoil-range

[OttawaTom]

I found this paper to be a really good discussion on gear on for old manual transmissions (it was written with a Corvair transmission in mind, but it still applies to any manual transmission):

https://www.widman.biz/Corvair/English/ ... saxle.html

It's fairly in-depth, but still understandable and informative.

I use NAPA GL-1 90W. I've used the Pennzoil synchromesh stuff before. I couldn't tell the difference when I switched to the NAPA, and besides it's too expensive for a leaky transmission

Based on that paper and your review, I'll look into the PennGrade stuff after I rebuild the transmission.

-Tom

[SteinOnkel]

Motul MotylGear

Use it in all of my cars, on the track or in stop-and-go it performs excellently.

[DieselSpider]

Dexron I or II ATF. I have an MSG5 Isuzu Transmission in my Spider and the manual transmissions in the rest all use Dexron I or II ATF also.

[gbsailing]

I was looking for GL-1 from my local oil distributor and he recommended DELO Gear ESI 80W-90. It meet the GL-5 GL-4 and MT-1 but will not damage the yellow metal in the transmission. I installed it in the trans and differential so I would only use one kind of gear lube. The cost was no more than the old style gear oil without the smell and you can get it in quarts. Gives the EP protection without the damage.

[hayesbd]

For what it's worth, I am running MT-90 from Red Line. This is a 75W90 GL-4 oil that works just fine and is not detrimental enough to "red" metals to cause any real issues. In my opinion, we can get too worried about the potential degradation of the brass synchromesh components and forget the fact that a straight 90 weight's high viscosity can cause the synchros plenty of wear and damage from trying to synchronize through sluggish oil. The behavior of the transmission when switching from GL-1 to this 75W90 GL-4 has been a night-and-day improvement with the lighter weight modern oil making things much more pleasant. I have very clear memories of driving my father's 124 with the original Fiat transmission fill and that car had very balky shifting (and I also recall Consumer Reports at the time making the same criticism).

I have been running this oil for many years and about 25,000 miles with no trouble on an already old transmission. It eliminated any tendency to grind while synchronizing.

I don't recommend any GL-5 oils which typically are very aggressive to brass, but sticking with old technology just because it was originally specified is not necessary. I would expect some of the other modern MT type oils could also work well; just make sure the additive package is not something known to attack copper-based metals.

Brian