I don't recall which Castrol brake fluid I used when I replaced the entire brake system in my restored 1971 Roadrunner. I believe it was silicone based because I remember buying it because one of it's benefits is it's supposed to be not as harmful for paint as the old style brake fluid. This fluid was used around 2002. Do you suppose it was probably DOT 4? My next question, is it compatable with DOT 5 Fluid which is now being produced?
Thanks for your help.

DOT 3 & 4 interchange. DOT 5 will not interchange with either.

you will have to COMPLETELY flush your brake system out before switching to DOT5

Thanks for the info on the compatability, I've got to think I used DOT 4. Any idea of when DOT 5 became commonplace?

DOT 3 & 4 are not silicone

DOT 4 is an improved version of 3 (both eat paint)

DOT 5 is silicone and wont eat paint

I dont know much about requirements for switching to DOT5. I put DOT5 in my car but I had the system completely apart and cleaned everything. I'm not sure if that was necessary or not.

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

Thanks mnguy55, I know what I put in was silicone based, I just had new paint applied and didn't want to ruin it w/brake fluid. Your data, combined with Clair_Davis' very comprehensive comments lead me to believe I used DOT 5. I didn't remember the purple color tint until I just read about it in Clairs post. Thanks very much Clair, perhaps I had the manufacturer wrong, but I do recall the fluid being a different color than the amber stuff I was used to. Thanks again, to you both, very much.