Silicone fluid is Dot 5, Dot 4 is just the latest version of the older fluid. On that note, mine has had Dot 5 in it for 15 years without any problem. I never tried any other fluid so I cannot say where the pedal requires more effort or not. I have read dozens of such stories about the fluid not working in older cars and trucks and all the problems folks have had with it, but mine works just fine for me. On the plus side, Dot 5 will not harm paint if thats something you are concerned with.
Joe
