Any gear oil with 'EP' in its designation, which includes GL4 &5 is rather more than 'hard on yellow metal' components. The EP additives get into the molecular structure of the surface of the component and prizes it apart on a microscopic level. A bit like ivy growing on a stone wall pulls the mortar out and eventually the wall crumbles. The result of this is that any wearing surface falls apart VERY quickly. I have seen 1mm wear in 1 hour of use on a not terribly loaded plain bronze bearing because of this. This is a serious problem if you get it wrong !
There are plenty of non-automotive uses for non EP gear oils (such as worm reduction gearboxes on electric motors on industrial machinery) so cast your net a bit wider than the local auto parts outlet. What you need is out there.
With regard to the viscosity of engine oil, check out the owners manual of your modern car, which I suggest should have more specific needs than a 50 year old MV. You will see that there is a huge range of acceptable grades of oil for a particular climate (more so in the UK than the more extreme summer/winter variation in Canada) so within limits you do have some latitude. Synthetic oil is much more resistant to breakdown than mineral oils but that is not a great advantage to MVs as you probably are not trying to get 16000miles between oil changes (I hope).
David
Last edited by David Herbert; 28-07-17 at 11:02.
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