The "Owner Operation and Maintenance Manual" manual for the 18609-18610 tiller calls for #90 gear lubricant:
I would be wary of using EP spec as it has additives that do not play nice with "Yellow Metals".
I would recommend GL1 spec 80-90WT which can be found in most auto parts stores.
Here's that manual in PDF form:
18609-18610 Tiller Oper-Maint.pdf