The difference is simple. The B58VT64N0 is made of black stainless steel and the B58VT68N0 is made of regular stainless steel. All other specs for the ovens are identical.
there are many de-greasers out there, or even lemon juice if you want a natural, food-safe chemical-free option.... for particularly bad build-up you could use paint stripper which is fine to use on enamel.
good old fashioned elbow grease with a scourer and strong solution of washing-up liquid will do it, but be patient.
