"I am considering replacing the mineral oil in my engine with a synthetic oil. The engine has 50,000 miles on it. I have heard that the mineral oil and synthetic oil are compatible. Is this true?"
Generally, the reference to synthetic oil for an engine means a lubricant is formulated with a polyalphaolefin (PAO) base oil. PAO, which is often called synthesized hydrocarbon, is pure and is compatible with mineral base oils.
However, because the PAO base oil does not dissolve additives effectively, it is usually formulated with an ester co-base (usually diester and/or polyolester). The additives are soluble with the ester, and the ester is soluble with the PAO.
Likewise, the PAO tends to cause seal shrinkage, and the ester causes seal swelling, so the effects are offset when both base oils are present. It is the ester that can cause problems when you change from mineral to synthetic.
Ester base oil used alongside PAO base oil in lubricant formulation has excellent natural detergency. In other words, it will clean up varnish on component surfaces as a result of thermal and oxidative degradation of the lubricant. When you switch from a typical mineral-based engine oil to a typical synthetic-based oil, the varnish layer will be removed by the ester in the synthetic oil and become suspended. This suspended material can rapidly clog filters, block oil flow passageways and lead to component starvation. The same is true for gearboxes and other industrial machines.
So think twice about switching to synthetic oils in applications where the engine or other machine has been operating for some time with mineral oils. If you decide to make the switch, try to clean the system before making the change and then monitor it carefully once you start it up.