A vacuum leak really affects the engine at an idle, unless it is a really large leak. As far as your question, based on where the vacuum leak is, it may or may not cause it all the time.
Your primary concern seems to lean toward the oil light. As the engine's RPM's drop, oil pressure will too. Therefore, it is normal for the pressure to drop as the engine slows. If it is running really low ready to die, the light could come on.
Now, having 1 1/2 quarts too much oil in the engine can cause leak problems, so I would recommend removing the excess. Also, I believe the manufacturer recommends 10W30 for your vehicle.
Finally, I still think you either have a vacuum leak issue, or the idle air control motor is bad. However, I can't be sure until the check engine light is on and you have the computer scanned for codes. If the light comes on, take it to a nationally recognized parts store and have them scan the computer. Most will do it for free. Also, keep in mind, if there really is no oil pressure, the engine will start ticking and knocking. If you hear that, shut it down ASAP.
Let me know what you find or if you have other questions.
Thursday, October 7th, 2010 AT 12:45 PM