pardon my lack of knowledge, but i just did an oil change about a month ago on my 92 525i and im getting a low oil level alert, i look on my dipstick and its on the lower quarter of the dipstick betweent eh two hash marks. i see no oil stains on the ground and it dosent smell like oil is coming out fo the exaust. when i stick my finger in the exaust pipe i get a little black oil, just enough so that it shows your fingerprint. other than add oil, what sort of diagnostic should i do to find the problem