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MG MGB Technical - DIFF OIL

Crawled under my 79 MGB to check it over for a trip we are taking. Had to add about 1/2 pint into the diff. Went to auto parts store to get grease gun cartridge for my new grease gun and looked at the different diff gear oils on the shelf.

The gear oil I have been using is 80W-90 weight. I did notice a few options available on the shelf:
75W-90
85W-140 for top up only (?)
75W-90 synthetic

There were perhaps a few other weight combos as well as non-synthetic.

I tend not to use synthetic in the diff and tranny due some reports of the synthetic damaging the synchros in the tranny.

What has me curious is why some of these oils are "top up only" especially on the heavier weight rating???

I was thinking about getting a heavier weight oil for the diff due to its age and mileage. It does tend to groan a bit when decelerating, but it has for the 14 years I've owned it. But again I'm skeptical about the synthetic and the "top up only" designation.

any thoughts on this

Cheers

Gary
79 MGB
gary hansen

Some of the additives in GL5 oils are not good for copper alloys and can lead to higher wear rates of the dif shims. Anything specified as GL4 is fine. Dif oil is all about pressure additives not viscosity. EP (extreme pressure) 90 is recommended for 1800 B axles, EP90B for V8's. The B denotes a higher pressure rating.
Allan Reeling

Gary, Have a look at this.
http://www.bestlubeusa.com/gearlube.pdf
Al
Allan Reeling

I think we've had this discussion before.

It can be confusing as to which oil to put in the 4-cylinder axle (and rack), less so for the V8.

The V8 uses gear oil of one type in the gearbox, and a higher pressure variant for the axle, albeit of the same viscosity. The 4-cylinder axle spec is the same as for the V8 axle.

These days the variants seem to be defined as GL4 and GL5, and Halfords for example makes the distinction quite straightforward as it labels one of its oils as 'Differential oil GL5' and the other as 'Gear oil GL4'. I've been using these for many years, even though the viscosities are slightly lighter.

It's using GL5 oil in gearboxes that seems to cause the problem, not GL5 in differentials.
Paul Hunt

Gary
I've been having a good run with Castrol SAFXA
It's a high performance 80/140 rear axle oil with the aditives for Ford l/s diffs already in it
It will go in both limited slip and non l/s
Designed for use in high load axles it really does the job
Example-- Our heavy duty Ford diffs here have hob cut gears and with normal HP90 oil when they get right up to temp (like after 100 ks on the highway) they get a bit groany but with the SAFXA - not a sound
If your axle has just a little gear noise on the overrun as you say, I reckon this oil would quieten it for you
willy
William Revit

If anyone is interested this "thesis" (not a big read!), provides a lot of answers.
http://www.widman.biz/uploads/Transaxle_oil.pdf
The main points are;
GL5 displays better EP wear characteristics than GL4. Hence would be better in Hypoid axles.
The higher sulpher/phosphur additives in GL5 WILL strip the copper alloys from synchro cones.
BUT the author does not refer to bronze in differentials, presumably because their use is pretty limited. However I would assume if GL5 is bad news for bronze in transmissions it would also apply to Salisbury shims.
75w90 etc., gear oils are NOT multigrades.
The use of heavier viscosity oils might be beneficial for the gears themselves but bad for bearings, particularly needle rollers, and penetration into tight spaces.
Allan Reeling

I use Brad Penn classic gear oil. It protects brass, bronze and copper components found in older vehicles.


http://www.penngrade1.com/Products/Gear-Lubricants/GL4-SAE-80W90.aspx



You can mail order it from Summit Racing:


http://www.summitracing.com/parts/bpo-023-7729
Steven Rechter

This subject was discussed in the T series TD-TF section under ‘Differential Gear Oil’ on 24 September. Loads more info there and worth a read.
J Hughes

I use Comma EP 75-90 GL4. Mineral oil, cheap and complying with specs for my banjo diff

regards

Jan
Jan Emil Kristoffersen

This thread was discussed between 01/10/2015 and 02/10/2015

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