Castro 30 Weight
I run 30 weight in my Opel GT and my other 5 cars, mostly Castro, but I will buy just about any major brand that is on sale as long as it is 30 weight.
I have not tried synthetic, and may try it when the GT gets a rebuild. To be honest, I have no condition which requires unusual protection other than high-mileage engines. My most demanding condition is the initial startup of an engine after it has sat idle for a month or so.
My cars are both foreign and domestic, 4 & 6 cylinders, 1972 through 1996. We have 6 cars; all but one has over 100,000 miles. Our climate is very mild, milder than Europe, no snow, no freezing, no hot temperatures. So we do not need the viscosity of a 5 weight. Occasionally we venture into the Great Central Valley of California which has high summer temperatures, but the 30 weight performs well. The most rigorous driving condition we face is stop & go traffic, and of course, starting the engine after a month of sitting idle.