That makes sense because I remember my '63 Ford Galaxie 500 XL back when I was 16 and I only knew about single W
oil so I used that. I was a mechanic at a used car lot and we had a 55 gallon drum of 30w Valvoline.  

medium. I have always used straight 40 W. in all my 550 plus minis.. later bc

No issues with the AT, it shifts good and the '68 only has 36,000 miles on it. I used Castrol 10w/40, but just wanted to be sure.
Thanks!  

In an SPI, the spec is 10W40 but you are also strongly advised to change it every 1500 miles vs every 3000.

Then again, I believe metro engines on a whole are advised to run 10W40, but I always run 20W50 in manual metro engines.

I wouldn't hesitate to try 20W50 in a high-mileage automatic that is showing issues when otherwise running 10W40.

With a classic Mini's auto trans, should the oil weight be 10w/40 or 20w/50?