Are you allowed to eat a fish hamburger at McDonald's, even though they use the same oil to fry the fish hamburger, as they used to fry the bacon?
All perfect praise be to Allah
From this Fatwa
It is permissible to eat fried meat and fish from these restaurants, subject to two conditions:
1. That it is not fried in the same oil as the non-halaal chicken or in vessels in which chicken was fried, until they have been washed, because the non-halaal chicken is maytah (dead meat) and is najis (impure).
2. The meat should be halaal.
ALso have a look at this Fatwa