I doubt there is enough difference between oil and sludge to make enough difference to measure. Too more factors are the metal particles filtered out are much heavier than the oil. I have found big differences from one filter to another in the same brand and number, Purolator L10193, one with 152 sqin media, and an earlier one with 185 sqin. It might be better to carefully top up a filter, giving the oil plenty of time to soak into the media and refilling until it stays full, weigh it, record the weight, and then do the same when you take it off.