1958 oldsmobile 371 cu,s engine. runs good no smoke Is is best to run a 30w for summer driving only or is the 10-30 oils ok for hot weather. car is not run in cold weather?
Is it still possible to get straight 30W? In any case, 10W-30 should work fine. In mild weather the two oils should behave exactly the same.
Incidentally, oils have improved greatly in the last 50 years. I wouldn’t worry too much about adequate lubrication with modern oils.
A '58 Olds? You’ll want a heavier oil than that. Maybe 20W50. Chances are the original recommended oil was a straight weight, but I don’t know what it called for off the top of my head.
I’d use either 30 weight or 10W-40 but be careful with the new API SM oils. They are formulated for the newest engines with roller type valve lifters, not flat face lifters. ZDDP content, needed for extreme pressure as is seen with flat face lifters, is very minimal in SM oils. ZDDP will poison a cat converter if your engine burns oil but of course, you have no converter in your 58 model. For a extra protection for your older engine you can mix in a quart or two of diesel engine oil to get the ZDDP needed.
I favor the straight thirty, especially when driving that specific car only in warm weather.
I would not worry.
First consider what 10W30 means. When the temperatures are cold that oil will act like 10W oil, that is it will be thinner so it will lubricate better. As the car warms up it will not thin down as much as a single weight oil so when the car is at operating temperatures it will act like a 30W oil, which happens to be what the car needs.
In the early days of multi weight oils quite a few additives had to be added to the oils and the result was less oil and less lubrication. They also tended to loose their multi-weight capabilities over time. All that is different today and the multi-weight oils lube very well and have a long life.
While single weight oils are still available and will not hurt your engine (as long as you remember to change oils when the weather changes) you can avoid that with modern multi-weight oils.
If my memory serves me correctly, in 1958 Oldsmobile engines suffered from camshaft failure and it was traced to certain brands of motor oil. It was an interesting problem, because some dealers were plagued with warranty work on camshafts and others didn’t have the problem. It turned out to be the type of oil used and I believe that Oldmobile issued a blanket statement against multi-viscosity oils. Now I hope that the oils have been improved in almost 50 years, but keep this in mind as you select an oil. I had an acquaintance who had the camshaft failure on a 1958 Oldmobile.