Don't even think about a 40 weight oil if you have over 1 mile as too thin.
CASTROL is CRAP, especially RS 10W/60, mobil not much better.
SILKOLENE PRO S 10W/50 is the best to use.
Look up AMSOIL TESTS which were done over 400,000 miles in one vehicle(semi) alone, very interesting reading.
tabetha