[ammonman]

The reason many experience new leaks after switching to synthetic oils are two-fold. First, they are much better at dissolving the varnish and other sludges created when the non-synthetic lubes degrade or are over-heated in combination with moisture. The synthetics keep the disolved contaminants suspended and prevent them re-depositing much better than non-synthetic lubes. This action starts removing accumulated deposits from cracks and other defects in O-rings, gaskets, seals, etc where the deposits have accumulated and sealed a microscopic leak before it can grow. Second, synthetics tend to"capture" and suspend the remaining non-synthetic lubricant. As a result, the non-synthetic lube that has impregnated the various seals, O-rings, and gaskets is drawn out by the synthetic lubricant. Depending on the material of the seal, the result is generally that the item swells further as the non-synthetic lubricant is somewhat quickly drawn out before collapsing back to the original dimension. After the swelling recedes, the item no longer seals properly due to wear. We experienced both phenomena 25 years ago in my industry when we switched all our rotary screw and piston compressor systems to 100% synthetic lubricants. We spent lots of money with various independent labs trying to understand why we suddenly had so many leaking machines across our network. The result was a lot of rebuilds where the only things replaced were bearings and seals before re-fill with synthetic lube and putting the machine back in service. With effective side-stream "kidney loop" style filtration in place, some of these machines have not needed an oil change since.