1990 Chevy Silverado Losing Oil

I have a Chevy Silverado V-6 with 89,000 miles. My truck has never used any oil until now. I had my oil changed at a drive thru oil change station as I always do. I drove 250 miles to visit family. On the way back oil pressure dropped below 30 and started to make a noise. I immediately pulled over and killed the engine. I checked the oil and there was none on the stick. It held 3 quarts of oil. I drove back home stopping to check the oil a couple of times. The oil stayed in operating range. I checked it before going to work and it was empty. I had the oil changed at a different place and checked the oil every morning for 2 weeks and the oil level did not change. So I thought maybe when the oil was changed the first time something was not tightened right and I lost the oil. I started only checking the oil every couple of days and one day noticed the oil pressure was lower than usual. It was out of oil again. I noticed the oil was being lost most often when I went on the Interstate and traveled at high speeds and not when I stayed at or under 45 mph. There is no oil on my concrete driveway. I don't see any oil on the engine. I have talked about this with a few other people and they say I am probable burning oil. The truck does not smoke. It puffs a little white smoke when it first cranks in the morning. Please help?