In reading the info on this site that describes the characteristics and processing of oil, it was mentioned that using a multi-viscosity oil with a viscosity range wider than necessary is not a good idea, due to the stress on the additives that are used to make the oil a multi-viscosity product. However, does the same guideline apply to synthetic oil? Castrol makes/made a version of Syntec that was rated at 5W-50; a really wide range. I used this oil in my 1962 Ford Falcon 6-cyl engine and when I would pull out the dipstick when the engine was hot, the oil seemed to have the consistency of water...really thin and "drippy". Not the way I would have expected a 50 weight oil to act under the same conditions. How does the recommendation about viscosity range selection apply to synthetics? What guidelines should be used when selecting a synthetic oil, especially if it will be used in vintage vehicles that were made before synthetics became available?