If I am not mistaken, I think I remember the Bentley specifically saying not to use DOT 5 due to silicon incompatibility with seals -- or maybe I saw it in a tech article somewhere else. Anyway, DOT 4 is more than adequate, so why dump something like DOT 5 which is questionable at best. I hear the ATE that ipd sells in mass quantities is a good DOT 4. I will probably try that next time around.

Looking at replies so far I see no mention of seal compatibility issues with the silicon DOT-5 fluid. But on the MGB board a while ago there was a big debate which ended with a post of a quote from AP Lockheed engineers advising strongly against changing over to DOT-5 due to seal swelling potential. I recall they stated that if you insisted on using DOT-5, a complete change of all rubber seals in the braking system was required.

Maybe it's a British thing? The natural rubber they used in the seals, or...?

Anyone know for sure if DOT-5 is totally compatible with the 240 braking system?

Certainly both clbolt and skip make very good points, and as for the Castrol LMA (DOT-4) mentioned by Cody, the "LMA" supposedly is an acronym for "Low Moisture Affinity" or something like that.
--
Bob (son's 81-244GL B21F, dtr's 83-244DL B23F, 'my' 94-944 B230FD; plus grocery-getter Dodge minivan, hobbycar MGB, and numerous old motorcycles)

Dot5 silicone fluid is great stuff on paper, but it sufferes from a unique flaw in reality. It can actually compress under extreme pressure, which is why racers backed off using it back in the 80s. It may not ever be an issue in street driving, but I'd rather stick with Castrol LMA Dot4.
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1991 245, 61k miles, looking for a 5 speed 92-93 245 cheap.

DOT 5 brake fluid is silicon based and has the main property of not absorbing moisture as DOT 3 and DOT 4 do. That is important if the car is a seldom driven restoration or the like. For a street driven car, I use DOT 4 which has a higher boiling point than DOT 3.

Skip
'93 850GLT
'83 242TI Flathood

Yes. Not much point, but yes. See other posts on the topic. Many experiences both bad and good (all good, personally). Flushing and filling with DOT 4 is cheaper. You can add DOT 5 directly to the system with no ill effects but will eventually need to flush out the old stuff (completely un-mixable, that is to say they separate) mostly because it will entrain moisture and cause rust when trapped under the silicone.
For severe duty with glycol based stuff try DOT 5.1 (similarly pricey).

Mike!