Got my Harbor Freight gas powered 1" impact wrench. However, the o-ring on the drain/fill plug for the gear box was damaged and the gear oil ran out during shipping. Owner's manual says fill the gear box with #10 gear oil. Never heard of it? Anyone know what it is? Googling turned up a similar question from an owner of a Chinese motor scooter. No one knew the answer but the dealer said use 5w20 oil. Any insight here?

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1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13

I buy air tool oil at Sears, Home Depot, Lowes. Not sure what the specific weight it is.

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'86 300E 5 speed
'71 Triumph TR6
'46 Cushman Scooter
'41 Ford 9N tractor

This particular impact wrench isn't powered by compressed air, but by a gasoline engine. He's asking about oil for the gearbox.

I'm suspecting that they are referring to SAE10, non-detergent motor oil.

The Farm & Tractor stores usually have that available in quart containers...

The stuff I find hard to locate is the SAE20, ND motor oil...that's the stuff to use on fan motor bearings (box fans, air circulators, etc...) and such...

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M. G. Burg
'10 - Dakota SXT - Daily Ride / ≈ 172.5K
.'76 - 450SLC - 107.024.12 / < .89.20 K
..'77 - 280E - 123.033.12 / > 128.20 K
...'67 - El Camino - 283ci / > 207.00 K
....'75 - Yamaha - 650XS / < 21.00 K
.....'87 - G20 Sportvan / > 206.00 K
......'85 - 4WINNS 160 I.O. / 140hp
.......'74 - Honda CT70 / Real 125
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“I didn’t really say everything I said.”
. . . . . . . . . . . . . . . . . . ~ Yogi Berra ~

mgburg is suspecting right with the 10w motor oil.

The dealer is about right with the 5w20 motor oil as well. 5w would be fine.
They have right around the same viscosity properties as a 75W gear lube.
I’d go with an SAE 75w gear oil over the 10w and 5w20 motor oils though.

The #10 is actually an ISO grade system number. They run 10 to 680. Often kinematic viscosity, not Saybolt. #10 is pretty thin for a gear lube. Lowest on the ISO chart. I’d say it should actually be about a 15 to 35 for your tool if you’re going to use it a lot.

It’s viscosity is based on Centipoise (CPS) or Millipascal (mPas) which converts to around 60 on the Saybolt Universal (SSU/sus) more often used with SAE numbers. No need to geek out about that stuff though. You’re at the bottom of their (SAE) lube chart as well.

SAE60w (non detergent) might be tricky to find, but SAE75W isn’t. SAE75 is about a #30 ISO number. Should work just fine. The bottom line is: You’re at the bottom of the charts for both ISO and SAE. And, 75w gear lube holds its properties up to 210deg.F as the Saybolt viscosity charts call out. I don’t see it getting that hot though.

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1980 300D - Veggie Burner !

you can only use it on the #10 gear.

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"It's normal for these things to empty your wallet and break your heart in the process."
2012 SLK 350
1987 420 SEL

woops, need to find my reading glasses. Thanks

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'86 300E 5 speed
'71 Triumph TR6
'46 Cushman Scooter
'41 Ford 9N tractor

Thanks for that explanation. Those Chinese manual writers were expecting too much of me. I'll look for some 75W gear lube. Does it smell as bad as the 90W? If so, I might consider motor oil because I can't stand the smell of gear oil.

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1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13

I hear you on the smell. If you think it's bad fresh, try it mixed with seawater and driven around for a while. (Somebody had a habit of backing down the ramp a little too far when putting a boat in the water.)

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Whoever said there's nothing more expensive than a cheap Mercedes never had a cheap Jaguar.

83 300D Turbo with manual conversion, early W126 vented front rotors and H4 headlights 400,xxx miles
08 Suzuki GSX-R600 M4 Slip-on 22,xxx miles
88 Jaguar XJS V12 94,xxx miles. Work in progress.