I worked at an auto parts store for a while which had a machine
shop in it. According to the machinist, when they quit producing leaded
gas there was enough additives used that the unleaded gasoline would not
hurt the older engines. This is an agriculture area so there are of older
engines designed for the leaded gas. As far as I know they didn't have a
problem with burned valves or seats. In Short unleaded gas is safe in any
older engine. You might want to check this out with machinists in your
area to see what they say.
I've been running unleaded in my 1942 Carryall with no problems
and if my information is correct didn't they use the same engine in the
M37? From the 40's up into the mid 60's ?
Hope this helps someone out. At least give you a reason to check with
someone who rebuilds engines for a living to get an expert opinion.
T Hall