1941 Century 66S transmission oil Update

[Lawrence Helfand]

Over the last six years of ownership I have refined the driving pleasure of my Century by trail and error. One issue I have had is the worn syncros due to poor maintenance decades ago. I found considerable water in the oil I first drained out. The shifting was rather stiff but got better with fresh 90w gear oil. Still was not happy with it especially when its hot. I tried every kind of oil settling on GL4 140 for the last couple of years. I was reading a post here that suggested Redline 75/140 synthetic so I ordered a gallon after reading many reviews and drove around in the 95 degree weather this weekend with the Redline and what a distinct improvement. Hands down the best shifting I have experienced in my 66S. Even in the heat it never got balky. Love it so much I filled my somewhat stiff Moto Guzzi motorcycle tranny and its never shifted better. Highly recommend it. 

Another fluid related improvement I made that had great results was refilling my shocks with 15 wt motorcycle fork

oil. Damping action was greatly improved. 

1. Red Line Synthetic Oil. 75W140 NS GL-5 Gear Oil

   www.redlineoil.com/75w140-ns-gl-5-gear-oil

   75W140NS GL-5 Gear Oil Contains extreme pressure additives like our 75W140 GL-5 oil, but lacks friction modifiers to balance slipperiness Popular for historic and vintage transmissions, like Mid-1930s to early-1950s synchro-equipped gearboxes in cars and trucks.

Edited August 2, 2021 by Lawrence Helfand (see edit history)

[EmTee]

Glad to hear - I put Red Line MT-90 in the Borg-Warner T10 in my '64 GP and likewise noticed a difference.  I recommend it whenever I get the opportunity. 

[ILIKECARS53]

I thought there were issues with using GL-5 in older vehicles because of yellow metal.   Thanks   Jim

[Lawrence Helfand]

Yellow metal? I read about that a few years ago and after finding swirls of metallic color I assumed was bronze in drained oil. I started using GL4/ 140 at that time. That said I started reading feedback about the Redline and how well it worked in worn prewar transmissions and differentials.  Redline is recommending its use in vintage cars and with that I thought it was worth trying. I plan to drain a sample out after a couple of hundred miles and check for any evidence of bronze. As I have been a fan of their products since the 1970s I trust they are aware of any issues with syncro material in vintage manual transmissions. I think the GL 5 warning was quite broad and might have more to do with a particular brand of oil. The Redline filled tranny now engages with less effort then anything I have used so I assume that means less friction and wear but this is hardly my area of expertise. I do know that many of the suggestions I have read here are just anecdotal BS passed around forever and accepted as gospel usually starting with My buddy's father was a mechanic and he said..etc.  As a longtime mechanic of cars and bikes myself I have learned to do my own research. I will post an update after I sample the oil and hopefully keep on using it.  

[ILIKECARS53]

Thanks for the clarification.  I will check out the RED LINE gear oil.   Jim

[Bloo]

GL-5 is a rating of suitability for hypoid gears, which are found in rear axles, not transmissions. A long time ago when there were less ways to make oil, it was fairly safe to assume a GL-5 oil was loaded with sulfur that would attack brass, and the GL-5 rating itself implied a level of slipperiness that might prevent transmission synchronizers from working properly. It was also fairly safe to assume that GL-4 would work OK in a transmission. Not anymore. It is a crapshoot. There is just no way to get around reading the oil's datasheet in 2021.

 

As for Redline, the oils they make that are appropriate and recommended for synchromesh are, in order of light to heavy, 1) MTL, 2) MT-85, 3) MT-90 (or 75W90NS), and 4) 75W140NS.

 

MTL, MT-85, and MT-90 are rated GL4 while 75W90NS, and 75W140NS are rated GL-5, not that it is terribly relevant when we are talking about a transmission. It isn't.

 

The NS oils are designed to work in transaxles that have both hypoid rear axle gears and transmission synchronizers in the same box. In a plain synchromesh transmission with no rear axle gears in it, you would probably pick MT-90 over 75W90NS if you were at the third thickness level. But then you might go to 75W140NS if you wanted the thickest one that is ok for synchromesh.

 

75W140NS is what I am using. None of them are very thick.

 

Don't use the Redline 75W90 or 75W140 oils without the NS in a synchromesh transmission. Those are for hypoid rear axles.

 

 

 

Edited August 4, 2021 by Bloo (see edit history)

[avgwarhawk]

I'm glad this worked for you.   I tried Redline in my 54 3 speed about 2 years ago .  I felt no discernable difference.  I since have replaced with regular Valvoline gear oil. I have learned to appreciate the Dynaflow in my 60 Electra.   But I do enjoy rowing through the gears for bit. No power steering or brakes.  Talk about road feel!      

[jameswulf]

I've had issues with the transmission shifting in my 1939 Lea-Francis Corsica Super Sports, which is sychronized in gears 2,3,4.  I tried a couple of different transmission fluids/oils, but was never able to really improve the gear clashing that was ever present when I brought the car over from England in April 2021.  After much searching online I finally changed to the Redline 75W140NS GL-5 Gear Oil as recommended by Lawrence H. and immediately experienced a huge improvement in the shifting. Now it feels like a true sychromesh and even the double clutch shift from first to second has been improved.  I'm now a true believer in the 75W140NS, so thank you Lawrence for sharing your experience.

Edited September 18, 2022 by jameswulf (see edit history)

[Lawrence Helfand]

Very happy to hear of your results James and congratulations on your rare and gorgeous acquisition. Is this the Frank Delaney car? Great low sporting stance. Guessing interior is blue leather? Reminds me a little of a Riley. Wondering what else is in your stable! 

Kind regards Lawrence