I run 15w40 in my 49 216. It stays somewhat clear for perhaps 100 miles then turns black. It is still somewhat translucent but barely.
One thing to keep in mind is that the color of the oil has no bearing on it's additive package and ability to clean and lube, it's more of an indicator of the condition of your engines internals.
These old powerplants were originally run w/non detergent oil which allowed the soot and other combustion byproducts to deposit and accumulate on the engines internals.
Modern detergent oil suspends these byproducts and carries them with it on oil changes. It also continually cleans the inside so it's normal for the detergent oils to turn black. It's an indication that the oil is doing it's job.
If you've ever owned/serviced diesels you'd have quickly discarded any concern over blackened oil
I'm fully in the multigrade/detergent oil camp in these old powerplants....
Dave
