[M.D. Holloway]

I went down a grade form 20W50 to 15W40 and found it to work fine. My temp held below 100C for a few long Tejas tours.

There is overlap as to the viscosity range that is considered acceptable according to SAE. The one characteristic that is most notable on viscosity when comparing synthetic to mineral based is that the initial viscosity of synthetics does run a little bit lower than that of mineral based. But more importantly, synthetics normally do not thin down when brought up to temp like a mineral oil will (see viscosity index).

The thicker oil is required for improved film strength. This doesn;t allow as much metal to metal contact. The typical way to increase film strength is to go thicker but that isnlt always the best opition. When you increase the viscosity you can also increase internal flow resistance which can actually build up heat. So, what to do...

New synthetic oil technology offers up some answers. With the advent of oil blending and further purification and additives, film strength can be imporved without having to increase viscosity.

Keep in mind, I tested several different batches of Mobil, Shell, Conoco and Chevron oils and found that in certain instances the reported viscosity weight was actually not correct. The fortunate side of this is that be going up or down one grade it has little effect on engine performance save a little on fuel milage and cold crank in Canada or the Northeast (then again that is very subject to many variables). Todays engines are very forgiving compared to those of the past.