Gear Box Oil

That would stop some clattering - as would some sawdust in it as well...

Gear lube is a 90 weight oil which is generally used in a standard transmission.

Automatic transmission is about a 10 weight oil hydraulic oil.

I guess 15W40 is better then nothing if that's all you have. But expect problems if used in either. It's too thin to provide wear protection in one and too thick as a hydraulic fluid in the other.

According to a chart I have (from Plant Engineering and probably others) SAE 90 GEAR oil overlaps the viscosity of both SAE 40 & 50 CRANKCASE oils. Yes, the scales are not the same between gear and crankcase lubes. However, the additives are different. So, go with whatever the manual or specifications say. (I have an old truck that wants SAE 140 gear oil.)

In a word, based on the general nature of your question, NO!

Not if you want to keep "using" it.

Do some reading on the subject of lubrication.

Fundamentals of Fluid Film Lubrication

gears like heavy weight, how long do you want them to last?

Read the manual. My old Explorer required automatic transmission fluid in the 5 speed manual transmission.

Think OP better take that as a "no", then.

Probably yes. For short periods of time. Short, as used here, needs to be defined by you...according to how much you value your car.

If you have an automatic transmission, you need lubricants that act as hydraulic oil (relatively low viscosity) but have certain additives that enable the transmission friction clutches to lock up properly. Also while lubricating the planetary gears in the transmission. And some bearings. What you don't want in auto trans oil is any EP additives because the transmission clutches will slip.

If you have a manual transmission, the problem is a little simpler because you are only lubricating gears and bearings (and some synchomesh clutches). These transmissions generally use a thicker oil with some extreme pressure additives that resist metal-to-metal contact at the gear tooth mesh.

15-40 oil is generally a heavy-duty engine oil, which has a different additive package suitable for heavy service in diesel engines. I don't believe it has EP additives. It does have additives that wash sludge off of the internal surfaces of the engine and keep it in suspension until the filter can take it out. It would also have additives to combat some of the acids that would form in oil due to the combustion processes in the engine.

If my only choice was to use 15-40 or nothing, I would use it, but make it a priority to talk to my oil supplier about the pros and cons of using that oil versus going to the correct oil.

Hope this helps,

Jon.

So, how will I know which is the correct oil? Is their some standard handbook or manual for selection of the oil?

Yep. It will be found, more often than not, in the glovebox.

You could go to your local automotive parts store and ask them. They could even put your vehicle information into their computer and give you exact specifications. Most transmissions that have a dipstick, have information stamped on it that show you the correct level. Some dipsticks also have the type of fluid that is recommended. Pay special attention to the level indicators as too much fluid is bad as well as not enough. If your vehicle is still drivable, drive it to the auto-parts store and ask them to help you with this process the first time so you can learn by an experienced teacher plus you would have the proper fluid type.

Generally no, but if your box is something exotic, it may do, or it may even be required. Google box brand / type / number, get instruction manual.

you must go to the makers hand book. Most gear boxes like a heavy oil specific for gear boxes but the old Mini ad Maxi from British Leyland ran very happily on engie oil such as the 15W40 you are suggesting.

You are getting answers here and you haven't even told what the gear box is from. How much guess work is that.

Dear Mr.dreamz,

The main deciding Criterion will be the Viscosity Matching for the specified duty, Working Temp., and if it is matching, it can be used.

Hence compare the Defined Specs for the related equipment and compare the oil Specs if matching, it can be used.

DHAYANANDHAN.S

When working on trucks IH had a 5 speed gear box that had a habit of seizing a gear to the shaft and the correction was to use an engine oil 10w-30 and end of problem, it was listed an an upgrade buletin to the dealer. In my drag racing days the MOPAR guys ran automatic transmission fluid in the manual gear box as well as the differential for faster shifting and less drag and again I did not see any negative effects. Some of them ran this combo year round and I did not hear of any issues. Of course you could add some moly to the gear oil for extreme pressure charistics.

I would go by what the manufacturer says (as other posts have noted). The original British Minis, late 1950s on, had oil system common to engine and gearbox, so it's not unknown to use engine oil for transmission. Presumably the gearbox was designed accordingly.

You asked a short question, here's a short answer.

Yes if it's an emergency and that's all you have.

No if it's not and the owner's manual says otherwise.