I just got a new air filter for this engine and it came with a pre-filter. The original did not have one. Do I use it or will it screw with the mixture?
I'd use it. I have two mowers with pre-filters (foam wrappers) and they do a pretty good job of catching big stuff. Don't forget to clean it every now and then (or per manufacturer's recommendation).
It won't mess with the mixture; they're pretty coarse and non-restrictive.
I just got a new air filter for this engine and it came with a pre-filter. The original did not have one. Do I use it or will it screw with the mixture?
Absolutely, I wash pre-filter with a small amount of dishwashing liquid & water hose, rinse thoroughly & squeeze dry. I inspect cartridge filter by holding up to sunlight looking from inside of filter outward after tapping filter against a solid surface to remove dirt etc.