I'm not big on oil additives, most don't live up to there claims and some can actually harm the motor. I know Caterpillar will not honor a warranty if some, like Lucas Upper Cylinder Lube, have been used. Some will increase the viscosity of your oil and can clog smaller oil passages.
That being said, I do like Seafoam. But then it is more of a cleaner than a lubricant. I can't remember if it says on the label but was always told to change the oil about 500 miles after using it.