I use 87 octane (regular) unleaded from a good nationwide station Chevron, Shell, Texaco, etc. Arco and Citco and all that other econo stuff is really inferior. My new cars have not run as good on it. So I figure a motor as unsophisticated as these might be more suseptable to issues from it (I read this as I write it thinking the opposite may be true now I'll have to compare). Also there is not only no benifit to using high octane on these motors it is not good for them in the long run.
High octane gas needs higher compression to burn properly, about 10:1. These motors are about 6:1 compression. Lower Octane Gas Is More Flammable at Lower Compression and Too Flammable (explosive i.e. preignition) at Higher Compression. Pretty much the reverse is true for high octane gas