The 310 was owned by a gentleman that had lost his medical and happened to be my neighbor who agreed to let me fly it for the cost of fuel and helping him detail it (he also had 2 sons that flew it). I was covered by the open pilot clause (we checked with AOPA insurance). I also had about 20 hours instruction in it. While NE of JLI VOR in bound to John Wayne, the engine oil pressure started to fall off the board. I assumed that the engine was going to quit any moment. After troubleshooting the system as much as possible, I had to feather it. Conditions were down to mins, winds were light, and there was some light mist. After declaring, I had to descend through a pretty darn thick marine layer while receiving vectors for the ILS. Scary. I • myself a thousand times over. I had about 500 hours at the time (about 30 in type). SoCal App and John Wayne were super cool and assisted me greatly. I was offered Palomar and a couple of other airports near by. With the performance and altitude I had, I decided to continue to KSNA. Ultimately I landed without incident and had to get a tow off the runway.