I finally checked in at the Simplicity "dealer" ( a good hardware store in Manchester).
The young counterman , using the computer, answered my query with "20-50 engine oil" , after I related Eaton 778.
He (the man behind the counter) did not think much of my ATF idea..."thats not even oil...ATF "...And ATF can be destructive in a transmission or engine not designed for this fluid..
So, its been a good day..Now I know what is meant by "all purpose oil" .
20-50, and I may substitute with 10-40.....or...
75 wt transmission oil, one very hard to find, recommended by Saab back in '69 and by our M88 , in one of their transmissions..
BITOG is most interesting, for those who "dig" oils.