I'd recommend sending in a oil sample for analysis. I buy 3 kits at a time from JG Lubrication. I've use a big syringe to pull a sample, the send a piece of tubing. Pull a sample, fill out the info, put the sticker on the bottle, send it in. Go to the JG site and use the number on the card to get results, they'll email the results. Last time I sent a sample in it took 4 days to get results.
2002 Monaco Windsor 38 PKD Cummins ISC 350 8.3L
2005 Jeep Grand Cherokee w/5.7 Hemi