I purchased my '76 M20F in December last year. At the pre-buy, my mechanic noticed that the engine was using a lot of oil...around 1 quart per hour of flight time. The current owner at the time strongly believed this to be 'within spec/tolerance'. We felt otherwise, but we were butting heads and he wouldn't lower the price on the aircraft.
I would have liked to get another $3K off towards the cost of the overhaul that we knew the motor needed. But it was a reasonable overall deal and the aircraft had 4100TT and 950SMOH. Prop had very low time on it. At the annual in Feb this year, we decided to go ahead with the top end field overhaul. Upon tear down, two pistons looked average with edge wear on the crowns so we replaced with new, and rings/seals on all four cylinders were replaced (along with a hone and cleanup of the heads).
Initially, there was expected blow by from 8 quarts to 7 at about the 7 hour mark, and then with more flying, the usage went down to 6.5 quarts. I put another quart in just to be safe. She has done about 25 hours now since the rebuild and has only had two quarts total added (including the initial 'top up' quart. She's currently sitting on 6 quarts so I'm just going to keep flying her and monitor the oil consumption, only adding oil if it continues to drop lower.
I cruise at no higher than 380F CHT on #3 which is the hottest cylinder. I cannot say as to why the aircraft was using oil excessively beforehand, but the top overhaul fixed things!