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Old 04-24-2021, 06:09 PM   #1
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Compare ATF and SAE 30 weight

I see any number of transmissions for which the manufacturers authorize either automatic tranny oil or straight 30 weight lube oil. I've always assumed that you would NEVER ever put ATF in an engine. Can anybody explain the properties of these two oils which allow 30 wt to be substituted for ATF in a tranny but not ATF in an engine?
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Old 04-24-2021, 06:36 PM   #2
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rgano,
ATF has high levels of additives that aren’t appropriate for engines.
But engine oils don’t have much or anything that would harm a transmission.
And I don’t know the viscosity of ATF but I’m sure they are similar. Probably a bit thinner at room temps and closer to 30w at running temps. The big difference is the high levels of detergents in ATF. In the 50’s we usta “flush” old car engines that had run for long periods w “non-detergent” oil. When a new thing comes along like radial ply tires, detergent oil or synthetic oil there are those that continue to use the old product. Sometimes the old product is better most of the time but at times almost necessary under extreme situations. Usually temperature and loads.

So I’m saying ATF used temporarily or mixed w regular lube oil for fairly short periods of time should be fine. Then go back to trans oil for transmissions and engine lube oil for engines as specified by the manufacturer of the product.
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Old 04-24-2021, 06:40 PM   #3
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Quote:
Originally Posted by Nomad Willy View Post
rgano,
ATF has high levels of additives that aren’t appropriate for engines.
But engine oils don’t have much or anything that would harm an engine.
And I don’t know the viscosity of ATF but I’m sure they are similar. Probably a bit thinner at room temps and closer to 30w at running temps. The big difference is the high levels of detergents in ATF. In the 50’s we usta “flush” old car engines that had run for long periods w “non-detergent” oil. When a new thing comes along like radial ply tires, detergent oil or synthetic oil there are those that continue to use the old product. Sometimes the old product is better most of the time but almost necessary under extreme situations.

So I’m saying ATF used temporarily or mixed w regular lube oil for fairly short periods of time should be fine.
Thanks. I too think the viscosities must be somewhere in each other's neighborhood. I remember running some ATF through my 1968 Jeep CJ5 to "clean it." My Ford-Lehman 120s ran on nothing by non-synthetic 30 wt.
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Old 04-25-2021, 09:59 PM   #4
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What kind of trannies? I have CAT 3208T/A. CAT recommends I use 30wt "gear" oil. You can get from CAT.
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Old 04-26-2021, 05:37 AM   #5
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ATF has very low viscosity compared to engine oil. Putting ATF in an engine would be much like engine oil diluted with diesel. And when that happens, bearings can get destroyed.

Do not put ATF in an engine.

Marine gears are very different machines. Bearings are rolling element, unlike journal bearings in engines. They can tolerate wide swings in viscosity.
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Old 04-26-2021, 07:28 AM   #6
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Most ATF is somewhere close to a 5W-20 in viscosity. And the additives have stuff that doesn't really benefit an engine, but it's also missing plenty of additives that are important in an engine. The whole thing about ATF "cleaning" an engine comes from 2 things: thinner oils clean better, and some older ATF formulas did have a decent cleaning effect, at least compared to the engine oils available at the time. With modern oils, ATF isn't really useful for cleaning.

For some transmissions, if they take multiple oils, they sometimes list a different input RPM limit depending on which oil you're using. So some applications for the trans may require one of the choices, while for other applications, either would be suitable.
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Old 04-27-2021, 05:56 AM   #7
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I put ATF in an old lawn mower engine and ran it just to see what would happen. It foamed up like crazy and came out the breather.
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Old 04-27-2021, 06:00 AM   #8
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If you need SAE30 gear oil, go to a CAT dealer and buy TDTO 30. It's reasonably-priced and made specifically for transmission use. They sell it in quart, gallon and 5 gallon containers.
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