Originally Posted by george3
I DIY so it cost me $20 - $25 for syn with filter and I use a good filter so I compare that to $30,000 plus for a Tacoma with options and too much is OK by me on the OCI.
Even with synthetic costing more than conventional, I still stick with the OEM change intervals.
Extended drain intervals are NOT the primary reason to run synthetic. They are pushed by Amsoil salesmen who are trying to justify the cost to customers, IE: Oil costs twice as much, but you actually SAVE the cost of the filter by doubling the OCI... But that is not the primary advantage of running synthetic.
Better cold-flow performance
Better film retention (benefits you every time you start the engine)
Better viscosity retention at elevated temperatures - the reduced level of viscosity improvers needed in synthetics are more stable in shear, and allow a synthetic 5w40 to maintain the viscosity performance of a 15w40 conventional (Diesel engines only, just an example)
Better resistance to coking - Coking is what forms the basis for sludge. It is also a critical feature for turbocharged engines, where the hot turbo is shut down and the oil flow stops and "cooks" in the bearings. Conventional oils will "coke" (not "cook") and leave carbon deposits which build up over time and lead to bearing failure.
#3 and #4 are the basis for the logic of running extended drain intervals.