What's the difference in gear oil & hypoid gear oil?
I've just got a 1987 4x4 with 4 sp manual trans.
Oweners manual suggest Hypoid gear oil in the frt & rear axles.
STANDARD TRANSMISSION LUBE for the transmission.
What's the difference?

 

Hypoid gears have a different oil requirements from std. transmissions. Most newer transmissions use an oil with less viscosity. Hypoid gears have a shearing action and need an oil that can withstand that.

 

Don't forget that wonderful smell that the hypoid gear oil's sulfurized extreme pressure additives have. (Well, some of us like it.)

Don is right: regular gears have primarily a "rolling" action on their teeth. Hypoid gears slide and the oil has to be able to stay between the gear teeth.

 

Correct me if I'm wrong but my understanding is that the high sulphur content of the hypoid gear lube makes it unsuitable for transmissions because it can corrode the gears, bearings, etc.

I like the smell of the hypoid gear oil also. Reminds me of the mechanic's garage when I was young and worked at an excavating company.

 

I'll start with "correct me if I am wrong" too. The steel in the bearings & gears in the rear end is not very different from the steel & bearings in transmission, so I doubt that this would be a problem. I used the same hypoid oil throughout my rear ends and transmissions for several decades until I started using synthetic oil in the transmissions, but had to keep using hypoid gear oil in the rear end.

Starting in the '70's manual transmissions began to be designed to use lighter oils to cut drag and increase economy. Some used ATF. These transmissions might not like the heavier weight oil that is common in most hypoid rear axles, but I do not think the extreme pressure additives are a problem.

Summary: everything should have the proper viscosity for its design, but hypoid gears NEED the extra film toughness to keep from wearing themselves out.

 

can the oil in the differential rear end and front transfer case be changed to lower weight oil than specs? will this increase mpg, etc. I have F250, 2004, 4wd, Harley Davidson.

 

Better follow factory specs to the letter on a vehicle under warrantee. Do read the specs in detail. They ususally list the Ford part number prominently, but in the fine print list the actual requirements such as weight, additive levels, etc. If there is a range of acceptible weights, use the lighter oil.

In general the only place that hypoid gears are used is in the front (4x4) and rear axles, so that is where you need hypoid-type gear oil. The transfer case is another set of gears similar to the transmission so it should be able to use the same oil. If you do not do heavy loads and long hot summer runs, you can probably get by with a slightly lighter oil and get better gas mileage. It is a trade-off - at very high mileages you may end up with wear. (In some cases, synchronizers may be picky about oil viscosity, so if shifting is not smooth, you may need to change back.)

 

It can corrode "yellow metals" (copper and its alloys)

 

I am not disagreeing, but I have run hypoid gear oil in a lot of transmissions that I have taken apart and never seen any signs of corrosion on the brass blocker rings on the synchros. Free sulfur and moisture can give you sulfuric acid, but I try to keep my transmissions dry.

I would think that these lubricants are compounded with additives to keep things under control. Now there are a lot of choices, but in the middle part of the last century, many shops used the same 90-weight hypoid gear oil in everything.

 

It can, but you need to be careful since lighter oil may not protect as well as the heavier one. Bacause of the CAFE requirements, today's cars are already using very light oils unheard of before (i.e. 5w20)

 

Here's a bit of reading:
http://www.noria.com/learning_center/category_article.asp?articleid=496
Look at the careful wording of some phrases.

In any case, this implies that corrosion of yellow metals was, and still can be an issue, though modern oils should have additives to eliminate or minimize that.

 

I agree, but I think a lot of that development happened a long time ago and that it still is going on. As I said, I am in agreement, but the questions were related to what one could do or not do. I personally made the jump to lighter, synthetic trans lub about 15 years ago. I think the reduced drag is worth the extra trouble to stock more than one gear oil. On the other hand, if I would need to put in gear oil in a trans or transfer case when only a hypoid gear (of the correct weight) was available I would not worry. (If I was an off-roader, I would worry about water causing sulfuric acid problems, even in the differentials, so I would change it out more often.)

 

It appears the answer may be "it depends." I did some searching and found this brochure from Pennzoil entitled "Pennzoil Automotive Gear oils: In A Nutshell."

http://www.stealth316.com/misc/pennz...oilsummary.pdf

On the second page it states:

The extreme pressure (EP) additives in most gear oils contain sulphur-phosphorus compounds that can be corrosive to certain "yellow metal" components, if present at levels higher than recommended by the equipment manufacturer. "Yellow metals", such as copper and brass can be found in transmission synchronizers.

The API (American Petroleum Institute) grades automotive gear oils using a scale that goes from GL-1 to GL-5. It appears that the higher the final number, the higher the concentration of EP additives. GL-1 is not for use in axles and contains no EP additives. GL-4 has 1/2 the EP additives of GL-%. The Pennzoil chart indicates that the GL-4 oils and not interchangeable with the GL-5. I would infer this is because of the higher concentration of EP additives in the GL-5 oils. So it looks like it depends on whether your manufacturer recommends GL-1, GL-4, or GL-5 for your transmission.

Back in the late 70's through the mid 80's when I worked at the excavating company we had one type of gear oil that came in a drum. It was used in all the heavy equipment, heavy trucks, and down to the pick-ups whenever gear oil was called for. If I changed my trans or rear end oil, that's what went in. But then again we didn't necessarily do what was right but what was quickest. Later on someone told me there was a difference between transmission oil and hypoid gear oil. Now all the manual transmissions I own use ATF.

The EP additives work. NASA tested turbine gear oil with EP additives and without and found that the oil with the additives had a significantly better fatigue life. Their research suggests the additives form a protective film from reaction with the surface. Pretty heavy reading but interesting.

http://ntrs.nasa.gov/archive/nasa/ca...1985004926.pdf

 

For what it is worth, designers of industrial gear boxes want the oil with the minimum amount of additives that will protect for the operating conditions. It is the oil that does the lubrication for rolling contact. To be simplistic, additives are used to prevent non-ideal conditions from ruining the oil. So if you have a vehicle transmission it will not need EP additives so the designer won't want them in there and may introduce materials that are not compatible with the EP additives.

 

I may stand to be corrected, but the service manual specs 80W-90 gear oil for my T-18 4spd.