What Castrol booster was it?
Adding stuff to fuel is not nearly as problematic as one might think, because many pilots flying at high altitude add stuff already. I add ~ 0.5% IPA when doing certain flights, as a precaution even though there is no known case of fuel icing on a TB20.
But the amount would have to be small in % terms. Even the 0.5% is a hassle to carry if you had to do it on every leg of say a 20hr trip. That's why the pros use EDME because that is only 0.15%.