If applicable for your year, I think from style, location, design the factory unit available from gasgas and seen on the isde model is a better unit, more surface area, more protected.
I know I'm pretty new here, you don't know I have a clue at all, but here ya go. Are you having an issue with your brakes? Brake fluid "fresh fluid" has no measurable "expansion" brake fluid is "hydroscopic" it will absorb moisture, by design, like a sponge, even in a perfectly sealed system. It is the moisture that builds up that causes nearly every internal failure of a braking system, as well as most performance oriented failures. That said, I am tickled my bike has the ISDE type extender, primarily for the additional volume as much as any potential for the additional surface area to dissipate heat. Plus it does look cool...........
I am a brake dragger by style, I'll never stop, many decades of that riding style from many years of drum systems....I have never had a brake fade from fluid concerns, I have seen a reduction in brake responsiveness/grabbiness from heat related aspects of the pad friction coeffecient most certainly from dragging.
Moneywise, I would suggest changing the brake fluid every year, preferrably before you put the bike out of commision for the winter if you are in that type area as you do not want the old brake fluid in your system stagnant on the seals of the caliper/master cylinder for any period of time just to keep everything warm and fuzzy, and she is good to go come the season.
Quality DOT 4 fluid has always met all my requirements, I actually favor the Mercedes Benz fluid as I find it extremely cost effective, readily available by the quart, and I use a bit of it changing the fluid in all my machines yearly. (Ialso know there aren't that many manufacturers of brake fluid and MB has some very unique standards that must be met, I know I'm getting the same stuff yearly) If you are doing just one bike, a different brand of a smaller container may serve you better as once you have opened the container, you should use it, do not let it sit capped and reuse it next year, I would recommend you only use remaining fluid for the duration of that service cycle and properly dispose of it at your next service interval as it will have absorbed moisture and lowered its boiling point given a year on the shelf.
If DOT 4, fresh, is not meeting your requirements and you are legit boiling your fluid, I would suggest considering MOTUL 600 it is compatible with your current setup and exceeds DOT 5.1 specifications, but is not cheap (course if one bottle fits your yearly needs, $20 a year is not horrible). That said, price wise I would try the 5.1 as the next step, unless I stumbled on some Motul at a great pricepoint, all can be augmented by the extender, but more is not always better, and you can change your fluid, which you should do regardless of what coolers/extenders you have, an awful many years for 50-80 dollars.
