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Diff Oil

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    Diff Oil

    So having searched the forum and done some digging I have the following info. that I would appreciate advice on.

    Open diff and car used for sometimes "spirited" road driving and gear changes.

    Original oil spec. appears to have been ESW-M2C-105A.

    This translates to GL-6 which is no longer used and so a GL-5 80w-90.

    My questions are:

    1. If I buy a GL-5 spec. oil do I still need to look for one that states EP? - or is that just marketing naming and all GL-5 spec. oils already contain extra pressure additives for bevel gears.
    2. Is there a reason that if I look for a semi-synthetic GL-5 I don't see any that state EP?
    3. Should the correct fill quantity be 2 litres (according to Haynes), or until it starts to drip out of the fill hole?

    Got my Sealy AK-47 at the ready and looking forward to seeing what comes out!

    '65 289 "A" Code GT Optioned Coupe, with 4spd Toploader and Arvinode exhaust - sounds great! :D :gears

    Gear oil in a 50 year old open diff doesn't need anything fancy. All gear oil contains a phosphate EP additive. Synthetics offer no advantages unless you are looking for extended drain inteval and can cause seal weep issues.

    Take your pick


      I got the impression from searching forums that GL4 is for open diffs, GL5 for split diffs (I use Westway GL5 EP90), but as previous member states 50 yr old equipment doesn't need anything fancy!


        Cheers guys - and yep, I appreciate it'll be happy with the right quantity rather than a particular quality, but at around a tenner a litre it might as well have what's best for it's future life.

        I know GL-4 is important for a gearbox, as the friction reducers in GL-5 can supposedly play havoc with the synchro rings - it was having seen all the diff oil options on the Opie site that I posted here.

        So a mineral GL-5 seems to be the way to go.

        Anyone comment on the fill quantity?

        ...which again likely has a fair tolerance, unlike my F-Type diff which was a 20ml range!
        '65 289 "A" Code GT Optioned Coupe, with 4spd Toploader and Arvinode exhaust - sounds great! :D :gears


          i just filled it to the fill plug, I think it was around 1 - 2 litre from memory


            When I filled up a dry diff (new posi unit) on a 9* it took a little over 2l. When I have drained and refilled its about 1.5 litres, probably because the diff has no drain plug so sucking out seems to leave a fair amount sitting in and around the diff as opposed to a cracked and apart fresh build.


              I used Comma EP90 GL-5 in my '67 open diff, pretty sure I bought 2 x 1L and still got half a bottle left so must have used about 1.5 litres. Just fill to level of the plug.

              '99 Bright Atlantic Blue Convertible (Stock) & '67 Acapulco Blue Fastback (393 Stroker)


                Thanks guys, appreciated.
                Have a good weekend all.
                '65 289 "A" Code GT Optioned Coupe, with 4spd Toploader and Arvinode exhaust - sounds great! :D :gears