Bro first of all 142ODO is not consider as high mileage for toyota cars. Few of our brothers are driving these car with 300k ODO with stock engine. Secondly, why do you want OD OFF on highways? Only you can use it for down shift to overtake or before applying brakes. and then turn on OD when gear is down or you have done overtaking.
BTW, whats the fuel mileage of your se saloon on highways and inside city.