Engines haven't changed much, oil has improved, so you can run 7000 miles between changes. If anything, a slant is more forgiving than current aluminum head overhead cam engines. What really matters is how much suspended carbon is in the oil, i.e. how black it gets. Diesel oils have more detergent, so some here run that (Rotella, etc). I use 10W30 (better mileage) non-synthetic and change it every 6 months in my daily drivers (~5000 mi). If your engine has worn rings (noticeable blow-by, burns oil, low compression), use 10W40 or even 20W50. The "high mileage" oils might be expensive snake oil.
