Oil is very dirty- I bought the car used so I have no Idea what mainterence (if any) was done previously

You can do that -I use tranny fluid a lot to cleanse it out-

I don't advise doing any kind of flush like that to a dirty engine if it has sludge in it. That is OK for an engine that is basically clean but has a stuck valve or something like that but if the engine is sludged, that sludge will loosen up and get caught in places where it can do a lot more damage than it is doing now. ATF has virtually no lubrication properties and you can do some serious engine damage experimenting with that stuff.