Some of the synthetics are better, but they have a temperature limit that is still too low for many pumping applications. It is for this same reason that we do not recommend putting any type oil between dual seals if we can avoid it. This method guarantees the bearing will be over lubricated. The only proper way to grease a bearing is to hand pack it full, but not the cavity where it is located. As the bearing heats up some of the grease will leak into the cavity reducing the amount of lubrication in the bearing.
Oil is easy to install and change.
Be sure you have an oil level indicator on your pump. Be sure the pump is level. Many pumps have been aligned without checking to see if they were level. The oil level should be half way through the bottom ball when the pump is at rest. Unfortunately you cannot use oil lubrication on a vertical installation.