I have a '96 Miata which I've been trying to track down a problem. I got it with 124,000 miles on it, previous history largely unknown. I have a 120-mile a day commute, and it hadn't been using any oil between changes. 12 miles of said commute is at 70MPH, the rest is 60-65MPH. The problem started when I drove to Houston (from Dallas) and back. It was mostly at night, so the speed limit was 65MPH. The car used half a quart on these trips. I have a fuel filter in the PCV tract to catch and remove the oil vapor and normally there are just black specs on the filter, now there was an ounce or so of oil. There are no leaks that I can spot (checked everything but the rear main seal) and no visible smoke from the tailpipe. I have not performed a compression or leakdown test yet. The oil used is 10W-30 and now 5W-30 Mobil 1. Any ideas on this one? It has me baffled with only using oil during extended trips.