Hidden among the oil thread banter was John's inquiry about brake fluid.

It is not good to mix glycol based brake fluids (DOT 3, 4, 5.1) with
silicone (DOT 5), but most import for any brake fluid is to flush from
time to time to get rid of moisture and contaminants which corrode
parts.

DOT 5 has a higher boiling point (with or without moisture present) and
thus is better for brake linings that may cause higher operating
temperatures.

DOT 5 is also easier on paint, but is slightly compressible (absorbs
some air) and thus more affected by high altitude operation, which could
give a bit of a spongy feel.

As for affecting seals, you may need to do some on-line research about
that.   Unless DOT 4 is hard on Dot 5 seals (your basic question), I
would run it a year or two.