There’s three factors to this: a design by Shimano which seems to be a lot more sensitive to fluid viscosity than other’s designs, ineffective bleeding of the brakes, and the viscosity of the fluid and how that changes with temperature. We can’t change the Shimano design and presumably you don’t want to replace your brakes. So let’s focus on the bleeding and viscosity.
Proper bleeding is critical to all hydraulic brakes and I address this at the very end.
The rest is about the hydraulic fluid which is something you can change that can reduce the problem especially when it’s worse at cooler temperatures. By using fluid with lower viscosity at cold temperatures, the fluid will move faster through the system. When it moves faster through the system, your levers are less likely to “pump up” (change bite point), at least during normal usage.
TLDR: Maxima Mineral Brake Oil is a safer and more widely available alternative brake fluid than Putoline. It’s the official SRAM mineral oil for their DB8 brakes, which use mineral oil instead of DOT fluid. I recommend using Maxima if you want to try a mineral based hydraulic fluid that will reduce wandering bite point on Shimano brakes. Engineering nerdery follows.
It’s important to understand that mineral oil actually isn’t really one thing, it’s a mixture of various length hydrocarbons, which is why the viscosity and other properties can and do vary quite a bit depending on how it’s processed.
Also, the mineral oil is only the base oil. The additives in it, even if they only amount to a few percent, can have a major influence on the properties of the complete fluid. So even though people commonly say the final fluid is “mineral oil” it is really a mix of some variant of mineral oil and additives. It would be better called “mineral base hydraulic fluid” to make that more apparent.
Typical mineral oil has a viscosity around 35-50 cSt. And it may have no additives or have perfume added (like in baby oil). We need mineral oil in Shimano brakes that has very low viscosity and the proper additives.
When we say viscosity in the context of brakes, we actually mean kinematic viscosity, and it varies with temperature. It’s often quoted at 40C / 104F (ISO Viscosity Grade) and 100C / 212F (SAE condition). But different mineral oils will change their viscosity differently with temperature, even if they have the same viscosity at 40C they can have different viscosity at 0C.
Pour point is a measure of the coldest temperature just before an oil stops flowing like a liquid.
Viscosity Index (VI) is a relative quantified of how much the viscosity of a fluid changes as temperature changes.
So if we want to replace Shimano fluid with something, we should use a mineral based oil that has the same or slightly lower viscosity at 40C and is not as affected by temperature, meaning it has a higher VI, lower pour point, lower viscosity at 100C, or all three. Plus we have to be careful that the boil point is high enough not to cause issues when the brakes get hot.
Oh and it had to have the right additives to play nice with the elastomers (“rubbers”) in the brake system and prevent corrosion, oxidation and foaming.
We can get some useful information from the legally required safety data sheets for the fluids.
Shimano mineral base hydraulic fluid has a viscosity of 8 cSt (1 mm2/s = 1 cSt) and a pour point of -35C per the safety data sheet. Shimano doesn’t tell us the viscosity at 100C nor the VI. The boil point is >200C and the flash point is >130C.
Putoline HPX R 2.5 shock fluid has a viscosity of 6.7 cSt at 40C, 2.9 cSt at 100C, VI of 425, flash point of 87C, pour point of -51C, boiling point not given. Strictly looking at viscosity it is lower than Shimano oil and will stay more viscous at cold temps. It’s a fork oil, so it probably will play nice with the elastomers but I’m concerned about the high temperature performance. That low flash point might also mean that it has a low boil point, which could cause issues in a brake application.
Now for a weird suggestion; let’s look at SRAM mineral oil. But wait! Don’t SRAM brakes use DOT fluid? Yes, but also no. There’s one model of SRAM brakes, DB8, which uses mineral oil, specifically Maxima mineral brake oil. It has a viscosity of 8.7 cSt at 40C, 2.8 cSt at 100C, pour point of < -57C, VI of 202, boiling point of 215C. And it’s specifically designed and validated for bicycle hydraulic brake use, albeit SRAM not Shimano brakes.
So I would recommend using the Maxima oil, not the Putoline. It’s safer to use and should still perform better than Shimano oil at low temperatures.
The final kicker is that the Putoline HPX R 2.5W is hard to find anywhere and costs $50 to ship to the US on top of the fluid price. Whereas the Maxima oil is readily available in US and I assume most of the world given it’s the official SRAM mineral oil.
The SRAM bleed instructions video is actually very thorough and the best I’ve seen. If you also ensure you get your brake hose vertical and tap it during bleed, then I think you’ll get the best possible bleed.