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Some of my vehicles use DOT 3 fluid and some DOT 4. Am I mistaken to assume that DOT 4 superceded DOT 3?
Technically they're all "current" though 3 is diminishing. Price almost follows the DOT level.
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Could there be a reason(s) to use #3 instead of #4?
No.
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If a vehicle specifies to use only DOT 3, can DOT 4 be substituted without harm?
Yes. So far as I know, the only manuals that would call out "only DOT 3" would be those written when new formulations were in flux.
DOT 3, 4, and 5.1 fluids are compatible. (But pick one & stick with it.) All absorb water to some degree, 3 the most, so all should be exchanged every few years. Water promotes corrosion in the system, lowers the boiling point (fade), and eventually will feel "spongy." The water gets in through pores in the seals, hoses, and reservoir and of course when the reservoir is opened.
They'll take your paint right off too.
Unless track racing stick to 3 or 4. DOT 3 can eat some seals, 4 won't, so if your spec says 3 you can use either. If it says 4 stick with 4 (or 5.1). 4 has a higher boiling point than 3, absorbs less water, and is compatible with all seals. Good stuff.
DOT 5.1 has a really high boiling point to go with its price. As far as I know that's its main advantage. That's why it's numerically higher than 5 while being closer in composition to 3 & 4. (DOT levels are based on wet/dry boiling points.)
DOT 5 is silicone based & you'll end up in the river if you put it in a non-DOT 5 system. Don't do it. I've never heard anything but that you should only use it in new, empty systems. A benefit is that it doesn't absorb water (that'll come in handy in the river) so its main use is for undriven collectibles.
Like most people, I drive cars with fluid that's several years past the several year interval. Haven't had anything cut loose yet but then I'm not racing.
I will admit, every time I reply about this one I seem to buy fresh fluid & then procrastinate the actual job.
David
p.s. buy a set of speed bleeders.