Aran is correct. Back then, there were no detergent additives in oils, and there is a good possibility that crud can get released back into circulation once the strong additives of the synthetics start acting. However, if the engine has been recently rebuilt, a high viscocity synthetic can be used safely. Synthetics with various grades of viscoity are available. I use 15w50 Mobil 1 on my 440's which has probably higher hot viscocity than the original oil used in the 50's hemis. D^2 Quoting "A. Foster" <monkeypuzzle1@xxxxxxx>: > Denis; > Unless your engine has been torn down, boiled out, and rebuilt, I would use > single weight conventional motor oil. Using synthetic on a 50 year old > engine is asking for trouble. For one thing synthetic is too thin for the > tolerances that are likely to be encountered. For another it might dissolve > some coagulated crud in the block and block an oil galley causing real > problems.