My opinion is that even with 138K on your motor, if you are not burning any oil and your engine seals are still good, you can switch to a high quality API SM-CF rated synthetic oil. If you are looking for a recommendation, I would use Amsoil 5W-30. It does everything better than petroleum, and it doesn't contain paraffin waxes which I like. I can help you get your hands on the Amsoil products you would need.

 

Have you heard about Pertua Oils? If not, here's a quick overview: PERTUA is a dynamic and innovative company established in 1988 to provide a scientific breakthrough in lubrication technology which offers numerous significant economic benefits and advantages. Its flagship invention, Pertua Oil and Metal Treatment is patented both in the U.S and in the Philippines.

They have APEX Synthethic Performance Gasoline Oil specifically for gasoline engines.