Switching to a Synthetic on new and used cars
DO NOT USE a fully synthetic until the engine has covered a little more mileage (approx 10k) because the engine is still bedding in for some time up to this point. Then switch to a good synthetic.
What are the benefits? Less wear, less power loss in the engine, better fuel consumption.
Generally, the reference to synthetic oil for an engine can mean a Group 3 Mineral oil. But if it means a lubricant is formulated with a polyalphaolefin (PAO) base oil, then it is pure and is compatible with mineral base oils.However, because the PAO base oil does not dissolve additives as well as a basic mineral oils, it is usually formulated with an ester co-base (usually di-ester and/or polyol ester). The additives are soluble with the ester and the ester is soluble with the PAO.
The PAO tends to cause seal shrinkage and the ester causes seal swelling, so the effects are offset when both base oils are present.
When switching to a fully synthetic on an older engine, or one with a higher mileage, it is the ester that can cause problems when one changes 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 deposits on component surfaces as a result of thermal and oxidative degradation of the lubricant. When one switches from typical mineral-based engine oils 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 and can potentially 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. It can be done but be aware of the potential for creating more harm than good.
This is also where oil seal leakage can be an issue as the beneficial layer of deposits on the seal are holding the oil in, but once cleaned off, the seal can initially shrink before settling again, so a short spell of oil leakage may result.
On rebuilt engines of the older type, such as the MGB, the same applies, allow for bedding with a standard oil, and then switch to a synthetic, although there is some concern that the higher detergency will not allow a beneficial depositing on seals and as such, the oil consumption may prove a problem.
