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The brake fluid is running low, so I need to add or change the brake fluid. We were at the Warehouse and they only had DOT 4 Castrol, but it says that it's also compatible with DOT 3. So, I bought it, but now I read on several forums that it's possible to use, but other forums disagree and say to stick with DOT 3.
What to do now? Can I mix it, or do I need to empty the entire system first, or do I need to return the stuff and get DOT 3?

DOT 4 is a higher grade than DOT 3 in that it has a higher boiling point.
DOT 3 and DOT 4 brake fluids are both polyglycol ether based (unlike DOT 5 which is silicon based). Therefore, DOT 3 and DOT 4 can be mixed together without problem.