Sounds like an excellent application of Esso XD-3 0W30. A full PAO group IV synthetic for well under $5, and in some cases, under $4/L. Buy it from a bulk Esso dealer, not Wal-Mart.
Mobil-1 and Castro put a lot of money into marketing their products, hence they are more expensive. XD-3 is very minimally marketed and generally targeted towards fleet accounts and industrial users. Its characteristics are very similar to Delvac-1 and German Castro 0W30, so it should be an excellent choice for your engine.