Amphibious aircraft pilot reported a drop in engine oil pressure in cruise. The pilot continued to destination airport and landed.

Narrative

I departed Runway XXR from ZZZ with myself and a friend on board my aircraft (Aircraft X) enroute to ZZZ1; via a direct flight plan with VFR flight following for our planned trip. On departure; ZZZ Departure had us turn right to an initial heading of 035 degrees to go North of the restricted airspace around and over ZZZ2 and East of ZZZ. It appears that ATC was going to take us down the interstate; past the restricted airspace to the East; and then turn us back on a VFR direct flight path to ZZZ1. However; my friend following us on the same flight path in Aircraft Y was given a left turn by ZZZ Departure to a heading of 200. I then contacted ZZZ Departure to ask for the right downwind departure; if possible; in order to save some flight time and keep us closer to the airport during what is usually considered a critical phase of flight. I made ZZZ Departure aware that I was familiar with the restricted airspace east of ZZZ and would remain clear and not violate that airspace. This was then approved by ZZZ Departure and thus I read back that approval and then proceeded into the right hand turn to execute the right downwind departure for XXR at ZZZ. The flight proceeded normally and we continued our climb towards our requested cruising altitude of 5500 ft. We passed over the river southeast of ZZZ and had reached a distance of around 5.5 miles from the field at around 5100 ft. MSL. This is where I believed that I noticed a slight drop in oil pressure. However; the oil pressure was still showing in the upper green arc limit on the display; so I didn't think anything of it at first and continued the climb towards 5500 ft. A few moments later; the pressure began to drop rapidly and an indication of such was shown on the display. I immediately disconnected the plane's autopilot system; regained manual control; made an immediate left turn back towards XXR at ZZZ and contacted Departure to advise them of the loss of oil pressure and the need for an immediate return to the field. I then consulted my immediate procedures checklist; for the airplane; in the event of loss of oil pressure and followed those procedures correctly. ZZZ Departure [requested priority handling] on our behalf; had emergency vehicles roll; and cleared us into Runway XXR for the return. Departure also instructed us to remain with them and that we did not require switching to the ZZZ Tower frequency as they would work me in to the runway. I then executed the immediate checklist review again and returned the aircraft successfully to Runway XXR. The engine began to grind as the remainder of the oil was being sent overboard out of the oil system and engine. We successfully landed and pulled off the runway onto the taxiway and up to the maintenance hangar. At this point; the engine begins to appear to seize. Maintenance personnel at the hangar take a look at the aircraft and note that the clamp around the oil reservoir of the system; from the oil change they performed the day before; was not on correctly and this was the source of where the oil was being ejected from the engine/system. They admitted negligence/fault at that moment (both Mechanic and later shop owner) as to the cause of the incident at that time.I am filing this report; as I have not received a phone call from any personnel from the FAA regarding this incident and have been made aware by the shop owner; that they have been dealing with an FAA Inspector. I'm filing it many days after the incident took place; as I didn't believe it fit the proper description for 'self reporting' as I (Pilot In Command) did not cause the incident or do anything incorrectly. I also filled out the appropriate paperwork and provided my FAA pilot certificate number and contact information on the forms required by emergency response at ZZZ that is required after an incident on the field. I have also contacted public safety and administrative personnel at ZZZ airport to obtain a copy of that paperwork and incident report;generated on their side; and I still have received no paperwork or call back. I'm concerned that this incident isn't being properly investigated and I request the proper personnel to look into this matter.