I’ve taken the view that a replacement for the Moss gearbox in my Mk2 has to offer a significant improvement in change quality, be a fairly straightforward swap, and not cost a fortune. I’ve recently obtained a Jaguar all-synchro complete with overdrive and bell housing. I’ll not be installing it for a long time, but the question arises as to what oil to use in the gearbox overdrive combination. I’ll try to summarise what I (don’t) know and what I’ve read below.

It’s important to keep in mind that gearboxes have very different requirements from engines. The oil is much more severely stirred and whisked around, smashed and ground, and, in the overdrive, submitted to large centrifugal forces. There are clutch surfaces that rely on friction and synchromesh baulk rings made of yellow metal. On the other hand, there are no products of combustion to deal with and no problems of being burnt in the combustion chamber or going down the exhaust.

I’ll refer to Laycock Service Manuals and some overdrive test results. There are links to find them further down the page.

Engine oil

Laycock recommended SAE 30 (or between SAE 20 and SAE 50 for suitable climates) engine oil. This matched with the Jaguar’s advice for the Moss gearbox, but not the later all synchromesh. With engine oil, it is important that it is detergent free. Due to the violent stirring that goes on, an oil with detergent can become a bubbly foam and bubbly liquids are close to impossible to pump, making the overdrive inoperable apart from anything else.

Multi-grade engine oil with viscosity improvers are also to be avoided. Gearboxes chew up and grind to bits the VI improver. A 20/50 oil can become thin quite quickly, too thin for the hydraulic system to generate pressure. A test widely reported on internet sites seems to support this.

EP gear oil

Jaguar recommended EP90 for the all-synchro box with overdrive. Ferrari indicated that Shell Spirax EP90 should be used in the gearbox and overdrive of the 250 GTE in spite of the Laycock Ferrari Service specifically instructing not to use oils wit EP additives – it confuses the Ferraristi. All this advice is from 60 years ago and EP additive packages must have changed since then. Still, in their service documents, Laycock specifically advised against use of EP lubricants. Their argument was that the large centrifugal forces could separate out the additive as sludge. Again, this appears consistent with the tests where, with EP90, the overdrive generated ever increasing pressure and would not disengage.

In addition, extreme pressure (EP) additives most often work by sacrificial protection. They attach themselves to the metal surface. They are subsequently ‘worn off’ the surface instead of the underlying steel. This is fine with steel, but if the metal is bronze, brass, or copper (yellow metal) it is likely to be softer than the deposit and be torn off with it, increasing the rate of wear.

Does the Jaguar all-synchro box with its brass baulk rings and helical gears need or benefit much from extreme pressure additives originally designed for spiral bevel gears?

For any type of oil, friction reducing additives are to be avoided as they will not be good for the clutch in the overdrive.

Manual transmission lubricant

I don’t think MTL (or MTF) existed as a separate category in the day. E-type and Morgan owners are very positive about its effect on the functionality of the Moss box synchromesh. Some are advertised as yellow metal safe. I have no idea what they contain.

Automatic transmission fluid

ATF, probably type F or type G (?), has been used in manual transmissions. Jaguar recommended it for the ex-Rover five-speed LT77 transmission when they adopted it for the series 3 XJ6. It seems appropriate for the overdrive. Someone suggested that I tried it in the Moss box of my Mk2 in the late 1970s and it worked very well, much as many have suggested that MTL does.

References:

Laycock service manuals: There are several around. They have similar but not identical content, presumably due to the different types of Laycock overdrive and some editing by the vehicle manufacturer.

https://static1.squarespace.com/stat.../Laycock+2.pdf

https://volvo1800pictures.com/docume...ck/laycook.pdf

Overdrive lubricant tests: These were performed by a company in the US called Quantum Mechanics. The results are quoted widely, but I can’t find the original report or make any guarantees about them.

Overdrives

Everyone and Glyn in particular, please give your suggestions preferably with explanations. Don’t be afraid to shout rubbish (politely).

 

I'm sure Glyn can correct me, but my understanding is that the all-syncro needs a gear oil with a GL4 service rating. The GL5 uses EP additives that are harmful to the yellow metals and the GL4 does not. Bevel gear mesh is quite different from helical gear. In a helical or straight gearset the the oil is compressed between the teeth - a fairly easy service to design for. In the bevel gears typically found in a differential, the teeth wipe the faces of the teeth and that wiping action tends to pull the oil off the teeth. In contrast the straight gears capture the oil film and keep it in place. The spiral gears are a much more severe type of service, and the EP ( extreme pressure) additives are the "last resort" type lubrication between the teeth to prevent metal to metal contact.

As it doesn't have those types of gears, the all-syncro doesn't need an EP rated oil.

 

There is only one product to use. A GL4 SAE 75W-90. It's in my rebuilt combo (sync 4, Compact OD) & perfect. It is readily available and provides excellent change quality over a broad ambient temperature range.

GL5 has too much Sulphur Phosphorus additive in it and will damage yellow metal Synchro cones at operating temperature. Different ad packs trigger between 90 & 120 deg C generally in GL5's. This does not apply to GL4.

Ignore other piffle spoken. Modern shear stable VI Improvers are used!




Note an official SAE 60 has been added to Engine Oils ~ This majority to accommodate a particularly hot running Porsche engine. There are now officially SAE 0W's available for flow reasons. ~ keeping up with official changes is a challenge.

 

I understand Aero 120 engine oil is the equivalent of SAE 60.
We used it in air-cooled radial and flat (opposed cylinder like boxer) aircraft engines in summer in tropical North Queensland where ambient temperatures are about 90F.
I also used it in a VW beetle which was my aerodrome car and it seemed quite happy. VWs ran very hot in that part of the world.
I understand the Australian air force used OMD 370 in their DH Caribous which I was told was equivalent to Aero 120

 

Thank you all for your responses, but I was hoping to provoke a discussion that went a bit deeper into the subject. I largely trust what the oil majors tell us about their products and their application, though that may need a little interpretation for old cars, so my choice is already a GL4 75W90. Unfortunately, most of the info from the big oilcos is use this and don't use that and very little substantial about why. I feel that many of us, that includes me, would benefit form a better understanding of the differences in the formulations of gear oils and how they work. It's general ignorance that allows the snake oil salesmen to get away with selling stuff based on claims that are pure, undiluted piffle.

Going back to my new gearbox, I've tried the change with it sitting on a bench and it feels very smooth and precise, certainly a generation or more on from my old Moss box on the basis of a side by side comparison. As noted in another thread, overcoming the reverse detente requires a significantly stronger thump. The box is a KCN, original to a 240. I think the ratios are the same as one of the E types. Bottom gear is much higher than in the JCN (OE in the larger engined cars), which makes it more GT and less suitable for pulling heavy trailers and extracting tree stumps. Also, the overdrive has fewer springs and operates at lower pressure. Some people who have replaced 2.4s with bigger engines say the 2.4 overdrive has been fine. Either way, uprating it shouldn't be a difficult problem. Though the KCN is relatively rare and in my view desirable, it is far from sort after, which brings the price down to that of a Moss box, about £5 per kilogramme.

 

All that, that does is open a can of worms Peter with no logical conclusion & general confusion by most less technical. i.e. their competence lies in other areas. e.g. a Chartered Accountant or Actuary.

e.g. We make an ISO 32 (strictly the low end of a 75W) MTF for a certain OEM that is Potassium Borate dosed. Operation by electrophoresis. Potassium Borate migrates to the part to be lubricated more quickly in lower viscosity fluids. It produces a far thicker film on such part & is not sacrificial. The gearbox is pressure fed by a pump in it's sump. They developed the gearbox & handed it over to the oil industry to lubricate instead of working with the Oilco's from day one with the lubricant as a component of the design. It is sold by the OEM as a Part Number.

That comment alone would confuse > 90% of the forum.

They originally factory filled with a range of off the shelf products & their customers suffered endless failures.

A better comment would be keep engine oils well away from gear oils as this causes air entrainment & foaming. In many cases right out of the breather.

 

We have learned this the hard way at MBWorld.

 

I understand your point. Sometimes even the technically qualified seem to have a block, like the engineers behind that gearbox you mention. I've had the experience (as an (in)expert witness) of explaining/teaching difficult engineering to lawyers: for the one that has the mind for it, there's a dozen or more that cannot and don't want to understand.

 

About to put new oil in my 4 speed with overdrive gearbox. I see that GL/4 75-90 is recommended. Any problem using fully synthetic oil? It looks like that is all that is available in GL/4 oils

It's going in a '66 S Type.

 

A Synthetic is ideal. ~ the best. High VI base oil + less troublesome VI Improver.

 

i think i bought some synthetic 75-90w at napa. and use it in the differential too, IIRC.

 

Has anyone removed the overdrive from this gearbox while the engine and gearbox are in the car? Looks like I've got a small leak between the gearbox and overdrive. I saw in another post that they can be operated on the MKII.

 

I run Castrol Syntrax Ltd Slip SAE 75W-140 in my S Type, dead silent, differential in hot SA. 90 thousand miles on it with meticulous rebuild to the book with fanatical attention to pinion bearing preload & crownwheel and pinion mesh.

 

 

Why does the word 'synthetic' worry classic car owners so much?

 

????????????????????????????? ~ Some ill informed chap told them it was too thin.

 

They don't understand Viscosity Index.

 

VI = The rate of change in viscosity for a given change in temperature. Petroleum based products thin at a greater rate with temperature rise than synthetics do.

 

It's all in the viscometrics. Castrol Syntrax Ltd Slip SAE 75W-140 is blended from the middle of the 75W range and just into the bottom of the SAE 140 range. Thus the nomenclature is correct.