Scottish will lose you if 2000' and below within 15-20 miles of Oban. I generally ask aircraft to report abeam Lochgilphead and then transfer to Oban in that position. If not too busy we will pass details to Oban so they are expecting your call..........VHF is line of sight and around the hills one cannot expect great coverage, the FIR sector has transmitters on Lowther hill SW Scotland, Craigowl hill near Leuchars and Windy head up on the moray coast.