If you did spill oil on the engine, it can take a pretty good while before it will burn off completely. The oil pressure readings you're getting sound great. If you are getting different oil pressure readings now, it could be because you used a different viscosity oil than before, which could be helping your oil pump. The pump is basically a little propeller that's driven by the engines timing, which is why when the throttle is applied the pressure rises, much in the same way as your voltmeter, which displays the current amount of electricity being produced to run every electrical system the car has but also maintains and refills the amount of electricity in your battery, so it'll start the next time you need it to.
To find an oil leak, get under your car again, as if you're gonna change the oil, and use a rag to clean all around the drain plug, oil pan, and oil filter, especially where the filter meets the block. If it's not obvious that you just cleaned up oil from where it shouldn't have been, the next part is check those recently cleaned spots again in a day or so.
If it's still clean you can rule out the most common places oil ever leaks from. It also wouldn't hurt to get yourself a good sized cardboard box, cut it or break it down, and place it directly beneath your engine where you park. If a drop of oil drips on that cardboard you won't be able to miss it, and if it does leak, the position of the drop on the cardboard will indicate where the oil is leaking from. On your next oil change, make sure the gasket on the drain-plug isn't cracked or really dried out, and always keep an eye on your oil levels several times within the first few days of performing any oil change.
Hope this was informative enough to help.