I remember when I use to own a 1977 D100 with a four speed A833OD. I can still picture the owner's manual instructing to use ATF. At the time I thought it was odd, but after some trial and error I concluded ATF was the correct lubricant to use in that particular transmission. Skip forward 42 years and so many different brands with different viscosity ratings, that it boggles the mind. The previous owner, of my ride, informed me he used a synthetic lubricant. Didn't tell me which brand or viscosity. Just that it was synthetic. After researching, I am questioning the use of ATF or to go long and install an actual synthetic GL4 gear oil. It has leaked down considerably, with the reverse detent leakage, that I thought now would be a good time to swap it out. The question is which brand and which viscosity since it rarely hits 90 degrees (F) here? Thoughts?