I spent a lot of time quite a while back trying to research this quandary.
Here are the basic issues I uncovered. It appears the Healey gearbox/ overdrive requires a lubricant which satisfies six criteria:
- Overdrives are operated hydraulically – i.e. pressure is what makes the overdrive operate – so the oil has to also act as a hydraulic fluid.
- The Overdrive has a wet clutch – i.e. there is a clutch which spends its life immersed in oil (like motorcycles)
- Synchromesh operates on the gears using friction – i.e. if you reduce the friction, (as in use a ‘friction modified oil’ ) – then the syncros won’t operate correctly
- Temperature is also an issue – typically Non synthetic engine oil is thinner at lower temperatures non synthetic than gear oil (not such an issue with synthetics)
- Pre BJ8 gearboxes have brass rather than steel syncros – and ‘older’ EP gearoils apparently have additives which attack the brass – (hence I believe the ‘traditional’ statement about using engine oil rather than gearoil – and the same comments made by the Penrite document you posted)
- The oil has to be capable of lubricating the gearbox – i.e. provide shear protection in an environment which is like a box full of eggbeaters and doesn’t use a filter – i.e. gearboxes aerate oil by their design. And – refer to point 1 above – no hydraulic system can operate successfully with aerated lubricant
I think the ‘gear oil versus engine oil’ issue is a bit of a red herring in 2001. The 1950’s – 60’s BMC ‘gearbox oil strategy” – I believe – was based on what was
“practical & commercial & available”- rather than purely technical issues…. and it doesn’t take into account the technological advancements made over the past 40 years – e.g. Synthetic oils. Would anyone seriously argue that their car handled better on the original fitment crossply tyres than on say Yokohama A008RS asymmetric directional radials today?
BMC said the Healey gearbox & overdrive use MINERAL engine oil. But as you also said – the exact same overdrive (which fits on the back of the gearbox – and uses the same oil as the gearbox) on a big Healey is also fitted to a Triumph – and guess what? Triumph say only use a gear oil…aaaaaarrgghhh
What is the answer? I asked everyone. You name a Healey racer or Australian / English Healey specialist – and I have either phoned or emailed them. I even emailed the manufacturer of the Healey overdrive to ask them why Austin Healey & Triumph gave contrary advice on oil for their product. And guess what? The correct answer is… that there is no correct magic single answer.
But – the best advice I can give from my research, is :
1. If you want to use mineral oil – Most experts agreed that Brass syncro gearboxes (e.g. BN1 etc.) should probably use engine oil. This is because SOME MINERAL gear oils have additives which can corrode some brass syncros – so to be safe – always use an engine oil if you have brass syncros or use a SYNTHETIC gearbox oil.
2. However, if you do use an engine oil – all experts agreed that you should NEVER use a ‘friction modified” engine oil. Because a syncro cone relies on friction – i.e. a syncros whole purpose is to ‘slow’ the gear, in order for the gear to be easily selected – i.e. it relies on friction to operate… . If the oil is too slippery (i.e. friction modified) – then guess what – the syncro won’t work – new oil and real slow graunchy gearchanges (do you like that word – I do – graunchy) !! That is the major reason many people use e.g. Penrite HPR (30 or 40) – a mineral engine oil which doesn’t have friction modifiers – in their gearboxes.
3. It was unanimous that lubricants have come a long way in the past 40 years. Just because the factory used a particular tyre 40 years ago – it doesn’t mean they would use the same tyres again today. My point – oils aren’t the same today as they were in the 1950’s. Synthetic oils were not in the mass market back then.
4. Engine oil tends to aerate more than gear oil – and tend to hold the bits of metal etc. in suspension (the properties of engine oil are designed to hold in suspension the by products of combustion – and remove them via a filter) Gear oils are designed for a Gear box which doesn’t have a filter. (overdrives have a ‘strainer – not a filter)
Much of this research pointed towards a synthetic gear oil as the answer.
My advice – read the stuff at www.redlineoil.com and make up your own mind, based on your own use of the car. Personally – in my fully rebuilt, using 95% brand new parts (all new gears, all new shafts, all new bearings, all new seals, all new syncros – only used the ‘original’ selector forks & 3/4th synchronizer & casing) gearbox in my Healey 3000 – and a fully rebuilt overdrive – the transformation achieved by switching to Redline MT90 gear oil was amazing – and that was after I tried 4 different brands of non synthetic engine oil . Redline is not cheap compared to Penrite HPR 30 (which I was using just prior to the change to Redline) – but believe me – neither was my gearbox/ overdrive……. I’ve been running Redline MT90 in my gearbox/overdrive for over 18 months now- and Redline 75W90 in the diff (was a Detroit locker – now Quaiffe) – it would take a lot for me to even consider changing brands.
I don’t work for Redline or sell Redline oil etc. etc.!!