I have been running detergent monograde oils, like Castrol HD30, in my classic car. Can I use a multi-grade?
Yes, there is no reason why you cannot switch to a 15w40 or 20w50. Just remember to choose a viscosity appropriate for your climate and engine (see chart above). Most 10w40 and lighter oils do not have sufficient levels of Zn and P, unless a motorcycle or racing oil with API ratings of SG, SH, or SJ. If you are running a straight weight oil in a race car that doesn’t need cold start protection, most monograde racing oils (like Brad Penn Racing 50wt) have lower detergency coupled with higher Zn and P, and are the obvious solution because of their higher flash points and sheer stability for cars run on the track hard, hot, and for extended periods of time under these conditions. These true straight weight racing oils do not have the needed detergency for extended use and should be changed after every race session and are not best for the street. Also, many race engines that use monograde 40, 50 or heavier weight oils also use sump or tank heaters to ensure the oil is already to temperatu
