brake fluid

A few corrections to above are required.

All brake fluids are synthetic (i.e. not from "minerals" occurring in nature), and always have been. Putting "synthetic" on the label was marketing genius, I think first by Valvoline. No "modern" brake fluids are petroleum based (perhaps in 1920, so don't flame me).

DOT 3, 4, and 5.1 fluids are glycol type. Downside is that glycols absorb moisture from the air which causes internal corrosion and lowers the boiling point (bad, pedal will go to floor). If you use them, flush in new fluid every 2 years or so, especially if you live in humid climates. All brake fluids are required to be able to mix (even silicone).

DOT 5 is a quality rating, not a fluid type. Until recently only silicone could meet this higher rating. When a glycol finally did, they required it be labelled "5.1" to avoid confusion, but it certainly confuses. Silicone has the advantage that it does not absorb moisture, so never requires flushing, unless you stupidly drip water into it. All kinds of misinformation spread about it, like that it could mystically cause more corrosion by not absorbing water. I recently removed a single pot MC I had used silicone for 20 years and it looked pristine inside (changed to dual MC). It causes no strange problems if mixed with glycol (test reports), though the jars warn not to (lawyers?). It won't degrade your seals, indeed some British cars in the 1950's required silicone since glycol attacked the brake seals. Most trailer queens use it because it won't ruin their $10K paint job. I use it because I like no maintenance.