found this also
In practice, you can go up in DOT rating as long as it is the same base material. Mainly regarding your comment about mixing glycol based DOT 3 and glycol based DOT 4 is half true/misleading. You can mix the two and it will suffice for a system needing the DOT 3 rating but will boil and therefore have moisture/system issues in a system requiring DOT 4. Why does that matter? You can flush a DOT 3 system and fill it with DOT 5.1, the DOT standard is just a specification for characteristics of the hydraulic fluid. – finleyarcher Sep 26 '17 at 19:51
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The glycol vs silicone is true because they don't actually mix, so you have two separate fluids acting in the system, which generally results in inconsistent hydraulic pressure as the fluids resist each other. It would be just like mixing engine
oil with water. – finleyarcher Sep 26 '17 at 19:52
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@finleyarcher I wondered whether that was the case. I have since heard of cases where the rubber s4als whcih 'prefer' one type disintigrate if they're used with the other. Eg on my morris traveller, it expects Glycol-based fluid. If I put silicone in (even if all new gear and fluid), the rubber seals might not like it. – user2808054 Feb 13 '19 at 10:22