I asked the guy who inspected my block and crank before I built the engine for my Saff this question. He said to use a 15w40 Semi if you can get it or 10w40 semi if not. Hes owned and built cosworths most of his life, and works as an engine builder.

Im running mine in on 15w40 Mineral oil at the minute, then I will be sticking to his advice and using 10w40 semi. Probably Magnetec.