If this is the case, have the vehicle checked by a qualified technician. The second letter is the service class designator, which has been sequentially assigned since the service classification system started. AAA recommends that motorists use the highest designated oil in their vehicles. Synthetic Oils and Oil Additives Synthetic engine oils possess performance characteristics that are more desirable than conventional engine oil. However, synthetics should still be changed at the same intervals as conventional oils.
Synthetic oils clean better than conventional oils because of their ability to hold more detergents.
However, the high cost of synthetic oils makes the benefits far less appealing. If cost is not a concern and you desire protection beyond normal driving conditions, synthetic oils may be for you. For the average driver, the money you save may pay for your next oil change or two. Don't Spend Money on Aftermarket Oil Additives that can Diminish Oil Protection Engine oil comes with additives already blended into the formula. These additives are included to enhance the oil's performance and to help meet the demands required by today's engines.