you could try the 'Y' which is a variable flow teat.
It is variable, and in my experience with Charlie and Lexie it is still a VERY slow teat, which makes them have to suck quite hard. (I was keen on this as I comped with EBM for 6 months with lexie one bottle of EBM a day, so I found it reassurring to know that the bottle would not be "an easy ride".

As such I managed to exclusively BF Lexie for 6 months, in conjunction with a bottle of EBM daily. (Kept me sane, LOL!)