What do you think about using a fleishig deep fryer at an event that hasn’t been used in a week with new oil for deep frying pareve things where dairy will be separately served? The event is a drive through where people will pick up pizza and then a few minutes later pick up something deep fried.
If we were to ignore the be’ein, then the rule is that a fleishig or milkhig keli eino ben yomo can be used lekhatchilah for something pareve, even if one is going to eat it with the other min. So, for example, you can use your fleishig soup pot to cook noodles that you will afterwards melt cheese on.
Given the be’ein issue here – if there genuinely is edible be’ein remnants on the grate, etc., then you have fleishig cooking directly with pareve, and you can’t eat the pareve things with milkhigs. That being said – I assume that there is easily 60x the gunk, so it is bateil once you cook it, and it can be argued that it isn’t אין מבטלין איסור לכתחליה because (1) it is heter, not issur and (2) you are just trying to use the fryer, you are not trying to mix up the gunk. Add to that that if you have used industrial cleaner on it, it is all pagum. So, assuming it is definitely 60, and you have cleaned it with a poisonous/pogem cleaner, I can definitely see being lenient if needed.