I hope someone can help me because my little brain is starting to hurt!

I usually use the Picadilly line from Hounslow Central but on the days when I can't be bothered to walk the extra distance to the tube station I get on the train to Waterloo instead. I was under the impression that Oyster isn't valid on South West Trains and because of this mixing of my journeys I've only used my Oyster for PAYG so far.

However, I am confused by a recorded message that I heard at Hounslow mainline station on Friday that said something like 'please note that Oyster pay as you go is not valid on any service from this station' thereby implying that an Oyster with a travelcard on it is valid.

Is that the case?

If so, how would anyone know if you've got a t/c or payg as there are no barriers or touch in/out pads at Waterloo or Hounslow and lastly, if an Oyster t/c is valid and I put one on my card, which zone would Hounslow mainline station be in (given that 2 of the 3 LU stations are in Z4 and the other is in Z5)

I'm going to lie down in a darkened room now...

A.