It would not be good idea to mix Dot 5 with either Dot 3 or 4. However, you can use Dot 3 and Dot 4 interchangeably). The piston seals could be damage as a result of this mixing. Additionally, in an old motorhome that has not started it's life with Dot 5, brake hose lining could be damaged as a result. During the early days of the disk brake recall for Bosch calipers, this Dot 5 mixing with Dot 3 was claimed to be part of the problem. I have heard from several mechanics that they saw a sludge come out of brake lines that during the recall campaign.
