Car: Lexus LS400 1997

Issue: I came across this paragraph from a company called Stoptech:

"One last note on the DOT ratings: Systems designed for a particular type of fluid (especially prior to the wide distribution and use of DOT 4 fluids) should continue to be filled with that fluid. For example, in a car that was delivered with DOT 3 fluid, the internal components of the system (seals, brake hoses, and fittings for example) were specifically designed and tested for compatibility with the chemical composition of DOT 3 fluid. Because the DOT 4 grade fluid typically contains a different chemical composition, compatibility of system components may be an issue."
http://stoptech.com/technical-support/technical-white-papers/brake-fluid

Unfortunately the link doesn't work. I found it on Toyota Nation: https://www.toyotanation.com/threads/which-brake-fluid-is-compatible-with-toyota-fluid.958809/

So what I think they're saying is DOT3,4,5.1 are compatible with each other but not old car components.

So if we take zero chances with our brakes, this is a concern for me with an old car. Especially as I can't find guaranteed new DOT3 anywhere in the UK from a reputable source. Part number is 00475-1BF03 for DOT3 and 08823-80112 for DOT4. But I supposedly can't use DOT4 anyway.

At the Toyota and Lexus dealers they only sell DOT5.1. Looking at it from a positive POV they could be thinking 'Our engineers don't see the issue, so stop worrying'. Or it could be one of those big buisness things where nobody has thought about people driving 30 year old cars.

I'm thinking about replacing most of the brake parts on the car, so I don't really want to use the wrong fluid and am wondering if I should just get the DOT3 from America or possibly Japan.

Thanks