"Synthetic" is nothing but advertising hype, pay no attention to it. Back in the 80's I worked in a development group for a large company which manufactured and held some of the original patents for "moisture resistant" DOT 4 Fluids. At that time ALL DOT 3,4 & 5.1 brake fluids on the market were synthetic. I doubt the technology has changed a lot since then. If the fluid is labeled DOT 3 or DOT 4 each batch has to pass a compatibility test (at lest it did then).
I always buy a name brand. At that time we faced stiff competition from off branded products which were made from inferior ingredients but still passed the DOT tests. For example all our fluids were formulated with distilled glycols where some of the off branded fluids used what distillers term bottoms glycol ethers.
