Gday all, bit if a question for u all, i think i have answered it myself but just checking any way here..... it is I want to change the brake fliud in my 07 sportster was looking at motuls DOT 4 fluid but it is 100%syn i dont think i can run this in my bike because i think harley oil is mineral based so i may go for a dot 4 castrol fluid. Is this the right way of thinking? if i rebuilt the brakes(new rubbers) i could use the motul 100% syn not a worry.  Thanks for any feed back.   Jim

Any fluid labelled Dot4 is compatible with each other.
Motul Dot4 is still Glycol based, but synthetic. Your confusing synthetic with Silicon (which is Dot5)

If in doubt flush  the system with metho and away you go, you can change your fluid.