What the Stihl Dealer said, isn't necessarily wrong, he just didn't explain it very well. 40 to 1 has less fuel per CC than 50 to 1. Right?
If the motor is designed to run on 50 to 1, and, If you don't tweak the carb, when you change to 40 to 1, then yes, theoretically, it's be lean, with the extra lubricant not being useful.
At 40 to 1, it's leaner, it's hotter combustion, but with more oil in the mix. What kind of oil? Wally World cheapest on the shelf? Premium Synthetic? A lot of variables.
But yes, I could see where they'd say that 40 to 1 runs lean.
IMHO, a Premium Synthetic is much more forgiving as to mix ratios.