100 lb in 1 barrel, which is 31 gallons. Works out to be 3.23 lb of oats per gallon. Over 15 lb of flaked oats in 5 gallons sounds ridiculous. Did you brew it?
Picked up about 3 lb of various hops I won in an auction from a LHBS that went out of business. Also got ~25 grain bins for the many variety of specialty malt I have, for a steal.
I use a large 300 micron filter. I am mostly concerned about hops in my plate chller. No issues with hops utilization due to its large size. 6" diameter and stands inside my 10 gallon pot.