Engine oil contamination

Isuzu Rodeo 1999, 97,000 miles, V-6 3.2 L, manual transmission 4 x 2. Since new I always used Mobil 1 synthetic oil. About 7,000 miles ago switched to Penzoil synthetic oil. It might be a coincidence but I noticed a reduction in the oil consumption (it was using 1 qt. Every 1,900 miles approx.). Since 5,000 miles ago it is not consuming any oil at all, actually the oil evel is increasing approximately 1 qt. Every 1,000 miles and it is loosing its viscosity. All the other engine fluids stay at their normal level. I suspect fuel (gas) is somehow getting into the oil. The engine check light is on, but the codes are one for the heater sensor and the other, I believe, is for the EGR valve (P1404). Engine starts and runs normal. I will appreciate your help. Thanks