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Manual Tranny/What kind of fluid?

Discussion in '1973-1991 K5 Blazer | Truck | Suburban' started by 87K54WD, Feb 28, 2001.

  1. 87K54WD

    87K54WD 1/2 ton status

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    I've got an 87 K5 with a manual transmission (M20 is the PPO code). It is a 4 speed with a creeper gear (L) then 1,2,3. I decided to change the fluid in the tranny and discovered that it was ATF (red) fluid. According to my owner's manual, it states that it is suppose to be 80-90W gear oil. The Haynes manual states the same unless it is the 4 speed with overdrive, then it is suppose to have ATF fluid. Well, I don't think I have the overdrive gear given that 3rd (really 4th gear) is my final gear and I have to shift into that gear after about 37-40 mph. According to the owner's manual, the M20 transmission is suppose to have gear oil. I bought this truck from my uncle the original owner almost 2 years ago. The only thing he had done was an overhaul on the clutch for normal wear and tear. I wonder if the shop put ATF fluid in by mistake. Is this bad for the tranny and should I change it back to gear oil. I'm willing to use Mobil ATF Synthetic, but I just wasn't sure if ATF was too thin for the manual tranny. Any ideas/suggestions is greatly appreciated. I'm eager to start using the K5 again. I replaced my u-joints, pinion seal, and changed my differential fluid recently. When I had the driveshaft out, that was when I noticed the ATF fluid instead of the gear oil. Thanks in advance K5ers!
     
  2. thatK30guy

    thatK30guy 1 ton status Premium Member

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    Stick with the 80-90 oil. They have used this oil way back when they were started. My service manuals also say the same thing - 80-90.
     
  3. jimmyjack

    jimmyjack 1/2 ton status

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    I'll second the motion. ATF is a hydraulic fluid more or less.

    creepin' and crawlin'

    Jim
     
  4. tom

    tom 1/2 ton status

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    You said you saw the red oil when you had the drive shaft out. When you pull the shaft, you see the ATF oil in the 208 / 241 transfer case. The 465 oil is different and is not exposed with the drive shaft out.
     
  5. TX Mudder

    TX Mudder 1/2 ton status

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    Use 80w-90,
    Do NOT use ATF.
    Quick tip - it's kind of hard to get to the filler hole on the side of the tranny. An easy way to fill your tranny is to remove the shift lever and pour into the hole. To do this, remove your shift boot and then press in and counter-clockwise on the ring around the shifter. Easy as pie.
    -- Mike
     
  6. LittleJimmy

    LittleJimmy 1/2 ton status

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    AS FAR AS I KNOW IT IS CALLED GEAR OIL. I USED TO HAVE A MANUAL AND THAT WAS ALL I PUT IN THERE. IF ALL ELSE FAILS CALL THE DEALER OR WHO EVER YOU GOT IT FROM, THEY SHOULD KNOW.
     
  7. muddin4fun

    muddin4fun 3/4 ton status

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    If the casing on your tranny is aluminum then you are supposed to use ATF fluid. If you put gear oil in an aluminum casing it will eat the casing. If it's not aluminum then use gear oil.

    [​IMG] Keep your feet dry [​IMG]
    <A target="_blank" HREF=http://muddin4fun.coloradok5.com>http://muddin4fun.coloradok5.com</A>
     

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