I'm not too familiar with the older Cat motors...my old stuff is mostly Doo with a Rupp and Bolens in the mix.
In 1972, Doo spec'd 20:1 ratio for their sleds and in 1973 they changed it to 40:1. There were no changes made to the engines, but Doo did come out with a new oil formulation in '73.
I've been using 40:1 in all my old sleds using Doo's current semi-synthetic oil with no problems.
Not sure how directly this applies to the older Cat motors but just offering up the info to "yellow" the waters.
If I find a nice older Cat to restore, I'm sure I'll stick with 40:1 like I run in all my older sleds.