Do you have any idea what safe is? With pushrod V-8s we talk in terms of 10 psi per 1000 rpm. The 15W will be lighter at normal operating temps.

I swapped a pushrod V-8 from 20-50 to 10-30 semi synth and gained nearly 200 rpm in idle speed. I didnt notice a huge drop in PSI, but was only monitoring it +/- 5 psi. Incredible result however. I dont touch 50W even for the race car anymore.