Oil consumption previously minimal but last two years approx 0.1litre per hour consumed when under 2/3 load or above. Minimal oil consumption if under 1/2 throtle (revs not certain). A puff of blue smoke evident when increasing revs. Difficult to detect during running due to white smoke - always been present - previous post on this noted. Thermostat opens at 60*C. No obstructions in suction or feed pipes. Engine temperature feels correct. Engine starts readily. No evidence of oil leakage. No oil in cooling water. No water in oil. Volvo Penta 15/40 oil used. Bukh DV20 circa 1983. Otherwise seems OK. Any thoughts?