I run my 4K-C on regular ULP with flashlube fuel additive. Its the one on the counter of nearly every servo. Don't pay $3.50 for a 50mL bottle, go down to Autobarn and pay $16.99 for a Litre.
I reckon I notice a difference with the Flashlube, doesn't stink as much when started, just a bit more power.
Fuel economy is still shit - 400km to 44L.
My factory original tank is 45L - or so says my KE55 owners manual.