You presume correctly, i was replying to you and it is indeed a mystery as to where your post has gone - maybe a moderator has hidden it?
I get what you're saying about the turbo putting the ECU into Limp Mode and causing the gearbox MIL to light now, i havae similar on my Honda at the moment due to some idiot topping the box with DW-1 instead of the correct fluid.
Generally i get 22-32mpg out of my S Type but that is on LPG so only 80p/L - equivalent to 40-60mpg roughly in terms of cost. As for life expectancy, my last petrol car was sold with 238k on the clock from new and still giving good performance, economy, power and not using any fluids. Traded that one against my Honda (142k from new and likely to do double that easily) although thanks to the aforementioned ATF problem, that's only doing ~16mpg or 10mpg less than the V6 Volvo with 238k did!
Either way the OP still hasn't responded to my first response about the ATF and codes.............