I'm in the habit of using the up/down manual mode when driving my 2010 Subaru Forester auto. I do it mainly in hilly areas, but also in everyday commuting. I've heard that this may shorten the life of the transmission. I've heard the saying "brakes are cheaper to replace than transmissions" more than once. Could you please advise?
No, the transmission can be used the way you use it without suffering because it's changed more than it might if it were left in auto mode.