Can someone give more details about gas fillings in T5 fluorescent lamps?
I know that the classic 4, 6, 8, 13 W lamps have argon fill, but I'm not sure about others.

There are 11, 15 and 16 W lamps which have the same length as 6and 8 W lamps.
And T5 HE and HO. Is the only diffrence in the electrodes or do they have a different gas filling pressure or even a different gas like krypton?
I know that T8 16, 32 and 51 W 'Eco'(Philips name) or 'ES' (Osram name) have xenon as the buffer gas, so maybe energy saving T5 tubes also have something other than argon. Even T5 HE and HO comes in a more recent energy saving version, like Philips Master TL5 ECO 25, 50, 73 W.

I just had a chance to play with some nice old Osram's T5 HE's and HO's, original German and Italian. I used my spectrometer to capture spectra from cold lamps (mercury not yet fully evaporated) to get the idea of what buffer gas fill is. Based on near-IR signature I see, a dense cluster of IR lines, I can say the fill is pure Ar, no Xe or Kr added. About the same that I see for small T5's and CFLs too. I remember reading somewhere too that the fill gas for T5 tubes is just plain argon. Note that some lightweight inert gas with high ionization potential as Ne or He may slip in unnoticed by this method, as it is not preferentially excited in the mix, but I did not see Ne pattern even around cathodes, where it can show up.

I don't have access to any newer 'economy' T5 tubes, so can not tell for them.

See, old school T5 13W runs at the same current as modern T5 HE 14W (170 milliamps), is about the same size and is almost mutually electrically compatible.

 

But 14W T5 HE aren't designed to be operated on the magnetic ballasts of 13W T5.