Pilot reported smoke from the propeller area of the left engine in cruise. The pilot elected to perform an inflight shut down of the left engine and diverted to make a precautionary landing.

Narrative

Narrative of [request for priority handling]; Day 0; ZZZ at approximately XA20I was flying the second leg of a 4-leg flight; originating in ZZZ3; destination was ZZZ4.This second leg was to be from ZZZ1 to ZZZ2.The flight was generally uneventful VFR. The intention was to visually follow the highway as it departed ZZZ eastbound; as this was a route I was familiar with; and terrain allowed a comfortable crossing at 11;500 ft.Upon approach to the ZZZ area; there was a layer of scattered clouds at approximately 9;000 ft. MSL; which complicated the visual acuity of the pass; and made it a marginal proposition. The decision process was to proceed south at 11;500 ft.; which would add approximately 20 minutes to the flight; or to climb. Ultimately; oxygen was used and a climb initiated to 13;500 ft.During the climb; and due to the aircraft's normally aspirated engines; it was decided to change the propeller setting from 2400 RPM to 2700 RPM to provide additional climb rate. The first sign of abnormality was during this climb; as the left propeller lagged the right in responding to the RPM change. The right immediately governed to 2700; while the left took approximately 10 seconds to match. It was a slow and steady rise in RPM; so it was not immediately obvious if this was an instrumentation error; propeller governor issue; or something else. Oil pressure and temp were normal and matched the RHE; and the engine temperature instruments all seemed normal.While crossing the area at 13;500 ft.; I happened to notice white smoke streaming from the area behind the left propeller spinner. I checked oil pressure and it was normal. The engine instrumentation was normal with no CLD warnings from the engine monitor instruments. After a brief moment of reflection; I decided on a precautionary shutdown of the engine and a return to ZZZ airport which I had just crossed over. The engine secured and feathered with no issue. The smoke abated after approximately 30 seconds; and an oil streak was observed on top of the cowling. Additionally a rumbling vibration; not unlike a fouled spark plug; was noted.I [requested priority handling] with ZZZ Center and turned to the airport. I was provided weather information and heading. An ordinary landing was achieved on Runway XX without incident. I lost radio communications with ZZZ while descending through approximately 10;000 ft. MSL; so simply switched to UNICOM and made position reports and indicated this was a single-engine landing.Taxiing was slow and difficult; and an Aircraft Y announced inbound with intention to land XX. I announced our clearance of Runway XX and began to taxi to the FBO ramp. Just as it was appearing that taxiing further would be difficult; I was called on UNICOM by the FBO and offered a tug. I shut down the aircraft and awaited the tug.I was able to inspect the left engine by opening the cowling; and discovered a large hole in the engine crankcase at the #6 cylinder position. The connecting rod was discovered lying further back on the engine crankcase; astride cylinders 2 and 4. A piece of the crankcase that contained the 12 o'clock cylinder base stud boss was lying on top of the rocker covers between cylinder 2 and 4. The sump was checked and 8 quarts of oil was verified; down approximately 2 quarts from our ZZZ1 departure. We secured the aircraft.No injuries were sustained; and the majority of damage is mechanical in nature to the engine itself. Upon reflection; I think a few things are worth noting with this incident.First; pilots are often hesitant to [request priority handling] for marginal or 'assured' situations. Our situation was never in doubt; but I requested early. This paid dividends as I later learned that the FBO was contacted by ZZZ Center to confirm my landing -- the FBO staff did not note my radio calls. This allowed ground support to Dispatch -- something I would have never considered as a benefit of requesting. I did not need to use my 91.3(b) authority; however; I wanted the added attention and service in the event that the aircraft's mechanical condition deteriorated further.Second; it is rather unsettling that the left engine on the aircraft was actively destroying itself and the resulting vibration was mostly imperceptible. This is a testament to the engine design; but also a concern in multi-engine aircraft I had not previously considered. I assumed that 'thrown rod' incidents would always include a loud bang and a cacophony of noise and vibration. This was calm; and given the mountain turbulence already surrounding us; barely perceptible. The power loss was not even noticeable due to the high altitude and our operating near 40% power. Had the flight been at night or under IFR; the smoke would not have been detected; and the engine would likely have seized before I took action; possibly preventing procedures from succeeding and making the situation more dangerous.Third; I have had minor mechanical mishaps in the past at towered airports; and this resulted in a lengthy interview that followed with the area FSDO. I understand that this is a recent policy; that all mechanical incidents must be investigated in depth. Because so much administrative time was wasted on prior trivialities (for example; a precautionary aborted takeoff due to an alternator going offline during the takeoff roll) -- I very strongly hesitated in [requesting priority handling] as the situation was benign and manageable. I think the policy of mandatory reports from Towers/agencies for mechanical aborts creates a chilling effect and is actually detrimental to safety; as it may cause a pilot to continue in a marginal mechanical situation 'for fear of the interview with the FAA Inspectors'. The inspectors I have spoken with in the past also seem perturbed by what amounts to busy-work and investigations of pilots who are acting pro-actively and with an abundance of caution.