Mine was 500+ bhp built by the best yb engine builder m.a.d that was ran on Titan 10w50 as recommended lol the book may say 5w but no one uses 5w as it's regarded as too thin for an old pinto lol even the oil man who sells oil recommends 10w50 or 10w60
A 10w oil protects a car down too -20 it don't get that cold in the uk! The 50 weight gives it a little more protection when the oil is up too temp as it's a little thicker than the 40 these are old engines thin oil just leaks out or burns off lol