I strongly recommend the 15w-40 type diesel oils. Many of these engines ran fine on Castrol GTX 20w-50 years ago but that was before the additive chemistry was changed and the Cx diesel rating dropped. Having too much oil (9 quarts is TOO MUCH) is worse than the 10w-40 oil they put in. Chevron makes one of the best followed by Mobil Devac and then Shell. They are usually price at $5-6.50/gal at Wal Mart.