I have a 87 Nissan Pathfinder 3.0 V6. When filling up with gas I topped up the oil (0.5L) and drove about 3000 miles before filling up with gas again. Upon checking the oil again I noticed the filler cap was missing!! I drove all that way with no filler cap. I know that no oil came out the filler hole because the engine was clean. I wonder what kind of crap entered my engine and if I did any damage. I plan to change the oil in the very near future. It was 5W30 Valvoline maxlife.