Both were non-
synthetic, one was 15W40 the other 10W30. Viscosity of the 15W40 was less than the Mobil 1 20W50 according to mobil website. If it were April I would have no issues going with 15W50 but going into winter I will stick with 10W30 until next spring. In the end neither of us are wrong, we will never drive these cars 100K miles, or at least not the cobra