Tell us about the build, a Magnum 270 cam is very good. Few cams that stood the test of time, that got it right the first time.
10w40 if the temp stays about 35-40. The 5w30 in the winter.
With 6 quart pans after barak in , add a quart of mobil 1 synthetic , 5 of the petro oil. The Mobil 1 synthetic is good stuff, after trying many different cutter oils on my lathe. Metal pressing to metal with tons of pressure, mobil 1 gets it done. The synthetic mixes well with into the other similar viscocities.