Bought some yesterday at Advance Auto parts on sale, buy one get one free. I have excessive oil pressure at start up even in the summer months of 85 psi with 20W 50 synthetic blend and want to try a different oil. This motor is a high performance Pontiac 474 (1970). Is the 5W 50 a good choice, especially during the colder weather? I do like to start and drive it occasionally during the winter when the roads are clear. It says on the bottle, "formulated for classic engines" Also, will I have to readjust the valve lash as the hydraulic lifters will be filled with a much thinner oil when cold and when the valve lash was previously set?