Oil Wt s & Brands?

[Eagle7]

We have always used 20w/50 mineral or 15w/50 synthetic wt oils from Valvoline and Mobil 1. Both seem to be fine in terms of heat and pressure. On recomendations from several engine builders - Tony Riveria, Jesse Prather - we don't use low viscosity oils.

Do you know why they make that recommendation?