My ’66 Volvo has a 4 speed manual transmission, a M40.  The manual recommends 80w gear oil.  All “modern” gear oil is GL5, I had read somewhere not to use GL5 in this transmission.  So I emailed ExxonMobil and ask about Mobil 1 75w90 in this old transmission.  I actually received an email back from one of the chemist. He said not to use any GL5 in my transmission because old transmissions like the M40 have copper and brass parts and GL5 erodes those soft metals.  So, I searched for a synthetic GL4 and thank goodness Amsoil still makes this stuff.   The fluid change was uneventful, I was amazed that the M40 only uses one quart of fluid, but it is only a little larger than a shoe box.   It has no filter and the drain and fill plugs are easy to access.  I did not take any photos of the M40, but I will post some I found on the internet.

Products used:

• One quart of Amsoil 75w90 synthetic gear oil