Another oil question

Hi all!

Quick question (and hoping for a quick answer and a peace of mind):

I checked my oil level yesterday and to my shock, it was just on the red MIN mark. I mean it was barely on it. What prompted me to check was the oil indicator light (yellow)that went on while I was driving. My oil pressure gauge behaved normally when this happened--3 when moving and 1.5-2 at idle or a stop. Should I worry about that I was driving my car while the oil level was on the MIN mark? Or the fact that my oil pressure gauge gave me a positive pressure reading is reason enough that I should NOT worry? Car is a 1990 190E. Thanks!

Oh, one more question. The place that I get my oil changed does this to my car while (or after) oil is being poured into my engine: The mechanic revs my engine; he says that is to create a vacuum for the oil filter or something. Hmmm, I think he also said it is to see if there are any leaks coming from the oil filter. Is this a normal procedure?