Additional reading on the matter doesn't lead to any definitive answers, mainly because the discussions tend to compare products throughout the range of brake fluids, from mineral oil to DOT 5.1.
So, I'll drill right to the underlying point. I have had increasing wheel cylinder leakage on my truck. I more suspect that this is a combination of age - the wheel cylinders were new in 2008 - and disuse. I drive it fairly regularly, but not for long nor very far. In the winter it will sit for more than a month at a time without being driven. The rest of the year maybe two or even three weeks occasionally.
The question about synthetic vs conventional DOT 3, if such a distinction exists chemically, is whether synthetic DOT 3 contributed to, or even caused, wheel cylinder leakage. Three noteworthy mentions are that I have never used any fluid but DOT 3, there is very little wear & tear on the wheel cylinders with 5k on the clock since they were new, and that the brake fluid has been completely flushed and rebled at least a couple times since.
