Much wrong info, but I'm not going to point to specific posts and upset people. DOT 4 can absorb more moisture than DOT 3, so is better. DOT 5.1 is even better and is the first glycol fluid which meets the DOT 5 spec. It is named 5.1 to distinguish from silicone DOT 5. They are all compatible. Ignore auto-counter "experts" who tell you that mixing DOT 3 & 5 will cause spooky problems. Of course best to first blow out all glycol fluid w/ air, then flush a little ethanol thru, then blow to evaporate that out. But, in my 1996 Voyager, I just flushed DOT 5 thru until I saw blue dye at each wheel. That is an ABS minivan and ABS still works fine so ignore the "lawyer-added" disclaimers on DOT 5 bottles ("not for use in ABS systems"). If worried about getting water into silicone fluid, then you should worry about that in your power steering system too. With silicone, you never need to flush again or worry about internal corrosion or removing paint from your pretty engine bay. The compressibility is no concern, but be careful to pour it slowly down the side of a funnel since any air bubbles take a long time to settle out. Re seals, some British cars in the 1950's required silicone fluid since glycol attacked their rubber. Silicone is much more benign than glycol. If you use glycol fluid, flush thru new fluid every 10 years in CA/AZ or every 4 years in the east, maybe 2 years on the Gulf Coast.
