DOT5 is silicone, and is tinted purple to let you know what it is while sitting in the MC.

DOT4 (and DOT3, and DOT5.1) are NOT silicone, and are all gold, amber, light yellow, etc., for the same reason.

You can add ANY DOT-rated brake fluid to your system at ANY TIME and it WILL function. DOT regulations require that all fluids be compatible from a mixture standpoint so you can add what you can get your hands on in an emergency situation. You will NOT get the full benefit of DOT5 or DOT5.1 fluids by mixing it with DOT4 or DOT3 fluids, but your brakes will work until you can do a full flush.

I don't recall any Castrol DOT5 fluid, but that doesn't mean it wasn't out there. Castrol DID make a DOT4 "LMA" fluid that supposedly had "Low Moisture Activity" or somthing like that. Good stuff, but still had a lower wet boiling point than DOT5 silicone fluid. DOT5.1 is not silicone, but has similar wet and dry boiling points as the silicone fluids. There are only a couple makers of true silicone DOT5 fluid, so any DOT5 you find is likely from one source. By DOT definition, any DOT rating is 100% compatible chemically and functionally with any other same DOT rating. That said, even if the fluid IS DOT5 silicone fluid, it should probably be fully flushed after 6 years. If it's not silicone fluid, the stuff in the system is moisture-saturated junk with a boiling point not much higher than water at this point. Do a full flush and get the system right before getting back on the road.

Clair