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Manual Transmission Fluid Viscosity


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#1 obsolete

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Posted 22 March 2020 - 12:21 AM

While doing research on manual transmission fluid/gear oil, I put together a list of the viscosities of a lot of the fluids I've seen people mention using in Starquest transmissions, and a few others. In case anyone ever wants this info, here's the list. First number is cSt @ 100C (212F, hotter than your transmission will probably ever get), second number is cSt @ 40C (104F). Higher numbers are thicker.
  • Royal Purple MAX GEAR 80W-90: 17.4/160
  • CRC Sta-Lube 24239 GL-4 85W-90: 17/136 (found on reseller websites, not directly from CRC)
  • Royal Purple MAX GEAR 75W-90: 16.5/100
  • Mystik JT-7 80W-90: 16.2/146
  • Red Line MT90: 15.5/86
  • Lucas 80W-90: 15.0/?
  • Valvoline 85W-90: 14.5/145.9
  • Chevron GL-4 85W-90: 14.2/134
  • Red Line MT-85: 12/62
  • Red Line MTL: 10.4/50.8
  • Mobilfluid 424: 9.3/55
  • Citgo TRANSGARD Tractor Hydraulic Fluid: 9.2/57.2
  • Pennzoil Synchromesh: 9.08/41.6
  • Valvoline MTF: 8.8/49.9
  • Royal Purple SYNCHROMAX: 7.5/39
  • Valvoline Dex/Merc ATF: 7.1/34.5

Edited by obsolete, 10 May 2020 - 06:06 PM.

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#2 Crazy Quest

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Posted 23 March 2020 - 10:52 AM

Just some input.

The Red Line MT90 really transformed the way my transmission shifted.

I would recommend to anyone that's interested in using it.
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#3 obsolete

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Posted 23 March 2020 - 11:44 AM

Cool, thanks for the info. Compared to the other ones on the list, MT90 has kind of a unique viscosity range; thicker than a conventional GL-4 80W-90 when it's hot, but on the thinner end when it's cold.
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#4 obsolete

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Posted 23 March 2020 - 01:24 PM

To save a trip to the store during quarantine, I am thinking I will end up filling mine with a cocktail of what I have on hand: mostly Sta-Lube 85W-90, thinned out with a little Valvoline MTF and ATF. I want to drive it next weekend.
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#5 obsolete

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Posted 02 April 2020 - 05:20 PM

Well, I've driven the car three times now since putting the new fluid in, which ended up being about a 50/50 mix of 85W-90 and ATF, and I was pretty disappointed in the beginning; the 2-3 shift I've been having problems with didn't seem to be any better. Rev it above 3000RPM in 2nd, try to get it up into 3rd, and wham, like hitting a wall, it just won't go in until the RPMs drop.

The third time I drove it, yesterday, it finally seemed to be getting better; I guess the new fluid is just taking time to work its way in between all the parts. It's still very notchy, but I can now ram it into 3rd from 2nd above 3000RPM and it goes in the first time! The car is much more fun to drive now. I am looking forward to putting more miles on it and hoping it will improve more as the weather warms up.

If I'm still not happy with it mid-summer, I may try something even thinner, like straight Synchromesh.
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#6 ucw458

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Posted 02 April 2020 - 06:38 PM

85-90w is too thick for where you live.  Not just in your SQ but likely all your other cars.  Cold temps cause gear oil to freeze into a wax like consistency.  I switched to Mobile one synthetic 75-90w in all my cars and noticed a big improvement.  Not just smoother shifting but decreased rolling resistance.  Especially in the cold temps.



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#7 obsolete

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Posted 03 April 2020 - 09:38 PM

Yep, I agree, that's why I chose to thin it out with ATF. I have M1 75W-90 in the rear diff and use it in other cars as well.
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