about a month ago weather was crazy cold so I started the '99 vw beetle with a 2.0 aeg engine and left it idle for about 20 minutes before I was ready to go. Drove about 10 miles on the highway and the oil light came on. The oil only had about 500 miles on it. I got the car with no history of what had been used in the car prior. When i changed it I used Mobil 1 synthetic 0w40. The oil light would go away when RPM's were less the 2000 but came back when over 2000 RPM's. When I got to a safe area to stop I checked the oil and it was "frothy" lots of bubbles. Fast forward a few days later checked the oil and it looked perfectly normal and the level was correct. Drove fine with no issues. Over the past few weeks put several hundred miles on it with no further issues. Today it was single digit weather again. started the VW and left it idle about 20 minutes while I cleared the snow. Took the bug for a ride and the oil light came on within a few miles. Brought it back home and the oil wasn't as Bubbly as last month but definitely had bubbles. Let it be for a couple hours while I went and cleared more snow. when I got home I checked the oil and it looked fin (no bubbles and correct level). Took it for a 5 mile drive with no problems- no oil light coming on. Any suggestions of what is going on? Is the 0w40 not right for it? should I be using 5w or 10w? The car runs great. It has about 118,500 miles.