I believe DOT 3, DOT 4, and DOT 5.1 are all interchangeable and you can use DOT 4. It's DOT 5 that you shouldn't use. I'm running DOT 4 in my brakes.

 

Is the friend a really old guy? There was a problem back in 1960s British cars, where only Girling DOT3 would work, and the seals would dissolve if you used American DOT3. I've read that DOT3 actually has a slightly lower affinity for water, than DOT4. I've used both interchangeably in all my (modern) cars with no issues.

 

No he's 28. Just wanted to run it by. And he's ASE, too. But I think it's okay now. I flushed all of the 3 before put in the 4.

 

I personally like dot 3 after using dot 4 and dot 5.1.
Dot 3 takes in more water which is bad however it holds more water without creating brake fade.
Dot 4 and Dot 5.1 takes in less water but it doesn't take much to create brake fade and therefore it needs to be flushed more often.

When I was using DOT 5.1 It was terrible in the cold and I believe it had something to do with the viscosity of the fluid being higher. It would show it's effects if you left it in there for more than a year.

Unless you race you will not be benefited by dot 4 so you are probably better with just sticking with Dot 3.

 

Here is something for you to read:

http://stoptech.com/technical-support/technical-white-papers/brake-fluid

Note that it is important to change/flush either at proper intervals.

 

Thanks for the info Carl. Had not read this before.