I did IST-OSL in TK J and I think it was either a 737 or A320. It had the newer 2-2 J class 'Barcalounger' types seats which are extremely comfortable for a narrow-body. There were only a few other pax in J so service was good too. I think that was about a 3 hour flight. Agreed you never really know what you are going to get with TK though - they are masters of fleet utilisation so swap aircraft at the last minute to get max utilisation out of them.