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I use synthetic in my car and truck, and would never use anything else anymore. Older tractors - now that's a different story. Engine tollerences in a tractor are much looser than in a newer car engine. Tractor engines normally run around say.. 160 degrees. Most thermostats in cars are in the range of 220. Car engines take much more of a beating than tractor engines. Having said this, I don't feel it matters one way or the other if you use synthetic in a tractor. Even a 10w mineral oil of today is far superior to the SAE 30 oils of 50 years ago. The synthetic oil would probably clean your engine more - which might explain why your oil pressure is lower. Less sludge usually means less pressure is developed when the pump tries to circulate the oil. One fallacy is the theory of higher oil pressure meaning better oil flow. This just isn't necessarily true. If your oil passages are gunked up, you darn well might be developing higher oil pressure. I really wasn't able to determine how long you have been using this oil in your tractor. But if it has been several months, and the the oil pressure recently dropped, I'd look for another reason besides the oil. Synthetics tend to flow easier and provide less viscosity friction when used in a tighter engine. This will be especially apparent in colder weather. That's why the car you speak of gets better mileage with synthetic oil. You probably won't see any difference with a tractor. Bottom line... if you don't like the lower oil pressure, and switching to a thicker oil restores some of the pressure, it may not mean that the oil is at fault. You might simply be covering up another problem.
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