Castrol GT LMA DOT 3 Brake Fluid compatibility ?

[Mark F.]

I have Castrol GT LMA DOT 3 Brake Fluid in my car now and need to bleed my brakes.

I’m having a hard time locating it in stock anywhere, so I have 3 questions:

❶ Would Castrol GT LMA DOT 4 be compatible with it when added in ? It seems to be more available.

More highly rated DOT 3 fluids include Bosch; STP; Lucas Oil; etc.
❷ Opinions on any of those (or other name brands) being better than Castrol ?

❸ Whether it just says “DOT 3” or “DOT 3 Synthetic” are both of those compatible w/ each other ?

Thanks in advance for any advice…

[Leif A.]

Mark,
DOT 3 and Dot 4 are compatible with each other...mix away. All standard brake fluid is synthetic. Brand is a personal choice...I, typically, buy the house brand. Although, there is barely a difference in price.

[Duke W.]

There is no generally accepted legal or technical definition of "synthetic" when it comes to automotive fluids, but as a general rule if they are not derived from traditional crude oil refining processes like distillation and catalytic cracking they are considered "synthetic". This includes lubricating oils blended predominantly from Group II and III (hydroprocessed) base stocks and/or Group IV polyalphaoelifins, and Group V esters.

All brake fluids, both plyglycol-based (DOT 3, 4. and 5.1) and DOT 5 silicone are manufactured by a complex petrochemical process using crude oil-derived base stocks, so all are "synthetic". So is antifreeze (ethylene or propylene glycol-based). Always take that word with a grain of salt.

Brake fluid performance specification are established by the US Department of Transportation (DOT), just like engine oil performance specifications are established by the American Pertroleum Institute (API). So if the package says DOT..., and it's from a reputable manufacturer or a reputable retailer house brand, then it's all the same, just like 15W-40 CK-4 engine oil is all the same regardless of brand. All the rest is just marketing hype.

DOT 5.1 has a higher wet and dry boiling point than DOT 4, which is yet higher than DOT 3. For the past several decades I've been using DOT 4 in all my cars except the SWC that has DOT 5 that I used when I did a complete brake system overhaul in the seventies starting with everything clean and bone dry because DOT 5 is not compatible/miscible with the others, and I only recommend using it if you start with everything clean and dry as I did, which means any new or rebuilt components must be disassembled, cleaned with denatured alcohol and dried to remove every last trace of glycol-based fluid.

The last time I bought a quart of DOT 4 brake fluid it was Walmart's "SuperTech" house brand because it was the least expensive DOT 4 on the shelf. I also bought a two gallon and one gallon jug of SuperTech 15W-40 CK-4 engine oil.

Duke

[Mark F.]

Hi Duke,

Very reassuring info - and I will not be starting from bone dry and clean, so DOT 3 or 4 is what it will be.

Also thanks for clearing the air on what "synthetic" means...