Hey
@FunctionalJnkieGrl 
I'm hoping you're staying functional!
This is really something that is impossible to advise on. Black Tar Heroin is also frequently contaminated with Fentanyl just like powder. It is far more likely if you're buying from the street that you're getting a product contaminated either partially or completely with Fentanyl or a similarly potent Opioid i.e. not Heroin/Morphine as one would want from Tar.
If you are making this decision purely for economical "maybe I should try this" reasons, I would suggest turning around and continuing with what you know. I feel you are too likely to buy adulterated drugs, jack your tolerance up to 11 over the course of a long weekend and then be stuck on Fentanyl.
The only way I know of that you're gonna be able to get legit Heroin of any kind is either from a trusted source that you test or via the darkweb. From the street, I highly advise against this.
If we're talking theoretically about "pure" BTH (Black Tar Heroin is essentially Heroin that has not been fully processed into powder form. It's not fully distilled, if you will) then Oxycodone and BTH are fairly similar in potency and duration.
At any rate, if you buy BTH you're not going to know the purity. You're going to have to test it out to see how it is and then go from there. Use a small amount first and go from there. That is the only way to figure it out. It would be impossible for us to map it out for you. Start slow and go slow.
Still, I advise against doing this at all. As an addict who has done this a million times, I can say that raising your dose is never the solution that we think it's going to be. You are always better off finding a solution outside of raising your dose.