yesterday I changed the oil in my 1988 vw Jetta semi race car. The used oil was AMS 20W50. My oil temp on steady 120KM/hour was around 90-100 C Now with 10W40 it is running closer to 110 C at same speed? My oil pressure has also decreased from 40 psi to 35 PSI. As I was pouring the 10W40 in my jetta it looked very thin. It was pouring like water. I changed the oil in my other car at same time but instead of 10W40 I used AMS 5W40. 5W40 looked much thicker than the 10W40. According to AMSOIL website 10W40 should be a little thicker.