My 06 lucerne(18,000k) uses about 28-30 oz oil every 3100 miles/4 months or so. I understand that is supposed to be normal operating conditions for these cars. My question is when should I top off when low. should I top off at 1/2 qt down or wait till a qt down. I plan on then running the miles out to around 4500k before another oil change. And which of my oils should I use. I presently have 5w-30 kendall gt1 syn blend in the car. My oil stash includes both cases of 5w-30 and 10w-30, of Napa Full Syn, Valvoline Maxlife, Castrol Syntec Blend,and Valvoline Conv. WB. I change the oil in my other two cars, why I have the stash, but can't get under the buick so I let firestone do it for less then 14.00 per oil change, and I just have to drive there and wait 20 min while having coffee.