Long term low level coolant contamination and fuel dilution can be very ugly. This is why UOA's are an important part of conditional maintenance verification. This vehicle a 1995 Chevy 1500 literally left the service bay with an oil change and came back 3 days later with a wee bit of deep thudding. Dropped the pan and found a spun bearing on the #5 rod and some tell-tale coolant leak sludge. We pulled a UOA just for grins...yup VERY ugly for 100 miles.