Cessna T210L pilot reported diverting to an alternate airport after experiencing power loss and low oil pressure at FL250. A safe landing ensued; and the flight continued after adding oil.

Narrative

I was flying a Cessna T210L that I just purchased. The pre-purchase inspection revealed oil leaks and compression leaking past the rings; however when speaking to one of the previous owners immediately prior to departure 'Oil consumption was negligible' and 'it blows the top quarts off if its full.' No advice was provided about avoiding long distance or high altitude flight. The dipstick indicated 8.25qts cold; and the owner provided me a quart for the trip home in-case I needed it.After an uneventful 3.5h flight to ZZZ. I topped off fuel; added the quart of oil; and the engine indicated 7.25 qts (hot); and I figured this was approximately a quart lower than true due to oil being spread throughout the engine instead of dropped down in the oil pan. Therefore; the oil consumption was between 1 and 2 quarts for the first flight. I took off from ZZZ and noticed a loss of 'normal' ~50PSI oil pressure after about 2 hours into flight while cruising IFR at FL250. The oil pressure was about 30PSI at the bottom of the 'normal' operating range. About 15 minutes later; I begin to notice a 1-2' manifold pressure fluctuation and 5-10PPH fuel flow fluctuation coinciding with the manifold pressure fluctuation. Turning on fuel pumps; switching tanks; and adjusting mixture or throttle did not solve the problem. Quite quickly ~5 minutes after the first noticed fluctuation; nearly total engine power was lost as manifold pressure and fuel flow plummeted.A distress call was made to ATC and I tried a variety of mixture/fuel/throttle settings; but none provided much power at that altitude. Without recovering significant engine power; I [requested ATC assistance]; began a rapid descent and ATC provided excellent services. I turned the prop to low RPM to provide less drag and chose a landing site. While in the descent to ZZZ1; oil pressure was above redline but very low; and engine temperatures plummeted. At lower altitudes; the engine responded with adequate power; but a gliding precautionary normal landing was conducted. During rollout and taxi; the oil pressure was back to the middle of green (~50PSI); so I elected to continue to taxi and shut down thinking that there was no permanent engine damage. After shutdown; I provided reports to airport personnel regarding the [incident].After filling the engine with 9 QTS of oil; maintenance at ZZZ1 cleaned the engine compartment and identified an oil leak around a STC'd Stratus C6LC-L oil filter adapter retaining through-bolt. The oil in the engine compartment and on the bottom of the aircraft was significantly worse than it was in ZZZ; and indicated a significant increase in the oil leak rate on the second flight. The oil adapter bolt was re-tightened and no other significant oil leaks were found. Ground checks were normal as well as the ability to make full power at field elevation. Upon the flight to ZZZ2; the engine became bootstrapped at 75% power at 13;500ft; possibly indicating turbocharger; wastegate; or turbo controller damage. I will continue to investigate as the aircraft maintenance schedule allows.I could've added more oil in ZZZ; and I could've grounded the aircraft in ZZZ for maintenance; however the data at that point didn't indicate such a high oil consumption rate. It's also possible that the cold temperatures at FL250 around the housing (-30F) were enough to shrink it while the through bolt was expanded from ~160F hot oil; thus resulting in an increase in oil leakage at that altitude & temperature. I could have cruised lower; however this would have resulted in a longer trip and more operation time.