Ok now are any of you have experience with these at all?
Back to old point though the mt90 was not good for this box
I have the āDrivenā brand in all my gearboxes that require 80W-90ā¦itās the best oil Iāve used.
If you only drive it in warm weather try 140W GL4
https://www.amazon.com/CRC-SL24228-GL-4-Multi-Purpose-Gear/dp/B000M8NZU2/ref=sr_1_3?dchild=1&keywords=140w+gl4+gear+oil&qid=1596300497&sr=8-3
The GL-5 question seems unsettled, though some think if a GL-5 is also rated MT1 that is safer.
But I wonder about that weightā¦
If it feels like a stick in the mud on a 94Ā° day what will feel like on a crisp 35Ā° morning?
Hi, Just drained my gearbox with 12k miles on it using Redline MT90 (66FHC). Thought I would try the new Ford manual transmission gearbox oil. List price was $30 something a quart, found it at Jegs for $16 a quart. Not very much quieter, but I found shifting was easier especially into reverse and first.
I think I will stick with this for a while as it only has 700 miles on it.
Regards,
Allen
By the way
ThankYou Everyone!
Gtjoey13-4
With love
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I donāt think you want any form of limited slip oil in your transmission. That oil you illustrated (Castrol Axle Limited Slip 80W-90 API GL-5) may be OK in your differential, but I wouldnāt put it in my gearbox (Moss or later all synchro). YMMV.
I have to wonder if using GL5 rated lube in my transmission for years is what caused it to fail?
I had tried a full synthetic and it made things worse. Later switched to GL4 , Brad Penn safe for yellow metals but It was too late. My thinking at the time was if GL4 was good a GL5 must be better, wrong, we live and learn.
Glenn
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The penn gl4 old school is on the way!!!
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How did it fail?
Iād expect the concern with limited slip oil would be its effect on synchros. The additives included in limited slip oil are specifically designed to eliminate chatter when the clutch pack is slipping ā meaning youāre driving around a corner. Time was when the only lubricant that worked was whale oil. Meanwhile, synchros require friction to function, so a lubricant that lubricates too well might cause the shifter to balk a bit trying to get into gear ā or worse, grind a bit.
I would not expect any of these lubricants to cause a gearbox to actually fail. I understand sulfur is bad for the bronze synchros, but I canāt say Iāve ever heard of a synchro failing because of it. The worst symptom Iād expect from using the incorrect oil might be that the gears grind a bit when shifting, or just donāt wanna shift. Fixed by changing the oil, no permanent harm done.
Note: If your gears grind when youāre trying to shift, the far more likely culprit is something made of rubber in the shift linkage that has gotten hard with age. Replace with a new, flexible part and the shifting will likely go back to good. Itās the first thing to look at, anyway.
My knowledge of transmissions is limited to my experiences with them. I have never torn one down, was just throwing out the thought that the wrong lube may have not helped things. This box was rebuilt about 15 years ago, with new bearings and synchros and layshaft replaced. More recently it shifted just fine and not grinding, it would pop out of first at times. It was just plain worn out I believe. First 3 gears very noisy 4th far less noise. My British car mechanic friend said not to take it on this last OIL LEAK trip, I did and was lucky to make it home. The racket from this box just got louder and louder with each mile. In the mornings going into reverse sounded like an animal in pain. I would often get the car moving in first and then go directly to 4th with less drama.
Glenn
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If itās like most classic 4-speeds, the first three gears are actually gears while 4th is actually a straight-through lockup of the input shaft to the output shaft, no gears involved.
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Interesting comment on the slippery aspect. Lubrication formulas have advanced since the eighties when I was a factory trained Jap motorcycle mechanic. Synthetic lubricants were verboten in roller bearing engines.
There was not enough friction for the rollers turn. They would just slide and get grooved.
Wow! is that ever a wild idea. Those would be like GL(?) 1100 Suzukis? It sounds like an almost and maybe interesting possibility. I do recall those bigger Suzukes had built up roller cranks. I can just imagine the problems weād have if the Jaguars were made that wayā¦
The castrol is drained just waiting on the penn gl4.
Looked great and no schrapnel in the oil. It might take an extra week to rip through the brass bit 
BUT This again was an interesting post because Ive got guys calling from around the globe.
Some using 20/50 vr 1 to 0/50 oils.
So ll stick with this and see Ive never changed so much gear box oil since working as a kid at the local shop.
Again, cold and at idle cold at a light, i can hear bearing sounding dry, or gear roll over on a newer car.
The old 80/90 back in it went quiet as a mouse cold or hot.
Yes it felt slicker in gear with the mt90 but to ME too slick , almost a metal to metal feeling.
I like the cushion feel or slight stick in the mud, it feels COATED, hard to explain, but thats the difference to me.
Kilbert was dead on, in 4th either way , its all good but in the first couple of gears it was WAY to loud.
gtjoey1314
Yes the GS 1000 and 1100 in particular. XS650 Yamaha also. As the engine, transmission and clutch are in the same case they also use the same oil. Synthetic oil could cause clutch slippage.
Rolling element bearings are a science unto themselves, and sometimes, their behavior is somewhat counterintuitive.
One of our engines at work uses rolling element bearings, and we were having failures due to skidding, and brinelling. The solution (approved by the bearing supplier) was to simply remove every other ball on every engine going forward, and increase the contact stressesā¦problem solved.
Boy howdy, and my idea of using recirculating ball bearings, on the con rods, rather than just good old, tried-and-true bronze guides, cost my Dadās and my prototype Bourke cycle engine its lifeā¦ and we never got back to it.
I live with that regret 40 years onā¦
Just an updateā¦Ive now done 350 miles with the penn 80/90
after a short drive I topped off the box and rammned a bit more fluid.
The shifts are fine and completely quiet.
Redline was a chatter box
Castrol worked but gl5 and didnt like the posi additive
The penn went in like glue and really, really made a big difference.
gtjoey1314
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Read this on the GL ratings and why some are a problem for our gearboxesā¦
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