Rick,
Check the melting point of that synthetic grease. We had a thread here just a month or three ago about the Singer lube vs petroleum jelly vs Tri-Flow grease. ( I don't know the title of it, nor did I bookmark it. My bad. )
In that thread our members either found a source or tested their own grease and found that the Singer motor lube and petroleum jelly melted at 115º. The melting temp of the synthetyic grease is higher. That is part of what makes the synthetic grease great for gears. It doesn't melt and it doesn't fling off like a lighter grease.
That's also what makes it unsuitable for motors with grease pots and wicks.
I don't know the repair shop you're talking about, but ( no offense intended ) I wouldn't believe them on this issue. That is just my belief due to some 45 + years of using oils, greases, and other lubes on various types of motors and bearings and other equipment.
Joe