What year engine? GM recommended detergent oil as an "option" prior to 1954, but specified multi-grade detergent in 1954.
If single-grade (and/or non-detergent) oil has been used, you could have sludge/gunk that could go into suspension if you change to multi-grade/detergent.
I'd remove the oil pan, clean the sludge out; and, remove the valve cover and see how much sludge is there (and try to clean it out).
