But wait. Aside from single viscosity offerings, Valvoline VR1 only comes in 20/50 or 10/30. Stock Pantera engines need zinc for the flat tappet cam and VR1 is one of the few oils made today that still contains zinc (they took zinc out of oil because it fouls catalytic converters). George noted that 20/50 has been the most commonly used oil in stock Pantera motors. It looks like I may just continue with 20/50. I have been using it for a long time without issues.