Been awhile since I've seen an FL820S, but the Amsoil filter looks smaller? Being a direct replacement though, I'd suspect they both hold the same amount? Anyone know by any chance?
Are you talking about the oil volume capacity of the actual filter can or the actual amount of contaminant the filter media can hold? The EaO will surely hold more contaminants and could go for a much longer interval by design. I have used the FL820S for 3-10,000 mile intervals and have seen the EaO used for 10-25k mile intervals. Both good filters.
As far as how much oil the can holds, the physical dimensions are the primary contributor to how much oil the oil filter can physically hold. Testing both using a scale is the only sure way to know.