I find that my ttrs use oil on long highway runs, but not much when off road. If you have just done 8-900km of ACT, then 3-400km of highway to get home, I would expect to top-up with oil. Maybe as much as 0.5L if you have not been topping-up smaller amounts daily.
Unfortunately it is just a feature of older air cooled 4 strokes, and the TTR does not have a large capacity sump so the oil goes round the engine twice as often as a bike with a 2L sump, getting hotter and breaking down faster as a result. (I see you are looking for an oil cooler, which I think is a good idea, but might not make much difference to the overall oil consumption) You could try a higher grade oil like 15w50 or even 20w50 fully synthetic in hot weather.
Have you done any of the Portugal TET? I am interested to know how it compares with the ACT.
