Similarly, in the zone of gland seals (including those of the microcracks, if any) will remove all the deposits and, in case of loss of elasticity of the seals, Synthetic oil, purifying yourself first 'road', will flow from the engine. Thus, the use of synthetic oils are not recommended in the following cases: * the presence of significant deposits on the inner surfaces of the engine * if the sealing elements (gaskets, valve stem seals, etc.) have lost elasticity and (or) have cracks (you need to replace the gaskets) * engines with seals with the 'stuffing' (GAZ Gazelle, UAZ, old Renault)? possible leak (although at extremely low temperatures, where no alternative is permitted to use 'synthetic') * in the running-in period for engines that require run-in, ie 'Useful wear' to break-in of friction couples. The same applies to the engine after overhaul. Brian Armstrong is full of insight into the issues. In these cases the run-in must be made on quality mineral oil, then you can go to the 'synthetic' * in the rotary-piston engines in all other cases the use of synthetic oils not only in no case will not harm even the 'old' and worn engine, but rather guarantees for its protection and ensure the best possible life. How to make the transition to the use of synthetic motor oil? If there is concern that the transition to 'synthetic' may have problems, you should use the following guidelines: First, to assess condition of the engine, ie check deposits and defective gland seals. If the engine oil leaks already have, then go to 'synthetic' is not possible to eliminate the reasons causing them.