Yep, take the oasis report and review it. It only tells the warranty work. Ask the seller if he had work performed outside work or what dealer he took it too. Ask seller if they can provide some kind of maintenance records as well as receipts.
I bought an 05 2.5 yrs ago w/112k at auction. After buying, I did the oasis report. It had seen little bit of warranty replacements. 118k EGR went bye-bye. 134k, the metal fuel (C&C truck) tank inner liner damaged the frame mounted fuel pump and couple injectors.
Over owning it for that time, I put nearly $6k into 'upgrades' and repairs. Upgrade went you can to repair. On 11/30, I traded it in for 1/2 worth for a hemi mage cab. I was able to pull my tunes and oil micro-filter bypass system off. I've sold the tuner (SCT) and looking for a buyer on the oil by-pass system.
If I was to go once again the 6.0l route, oasis report, receipts for maintenance record, and for me, the offer would be half its worth. I would be surprised the seller didn't keep any records of work performed.
Recommendation, when changing oil, change fuel filters.