A lot of the old axioms about changing fluids was to get rid of contaminants that build up in the fluids and don't necessarily get filtered out. This would include metal shavings or soot in motor oil, metal shavings in transmissions, or basically anything that ends up in power steering fluid such as wear metals or whatever flakes off the hoses. Coolant usually remains in the block, even with a block drain plug. And of course there's water absorbed by brake fluid.
Of course that runs counter to many the modern fluids that run longer with longer change intervals or where they aren't specified at all. Some transmissions specify a "lifetime" fluid. However, every automatic transmission I've ever heard of leaves most of the fluid in the torque converter after a simple drain, and some owners are almost offended that isn't not at least 80% of the old fluid drained. A lot of people just do partial changes for power steering fluid because of the possibility of introducing air in the system that takes forever to bleed out. DIY coolant drains are partial. As a result, some businesses advertise changes using full-exchange equipment.
Maybe we get all warm and fuzzy about getting out as much previous fluid as possible. I probably don't need to, but when I do oil changes at home I'm not in a hurry and let it drip for up to a half hour. Even after all that the color is still a bit dark (probably about 80-90% of the oil), but I know I did what I needed. Some carmakers even specify filter changes every other oil changes, which means more "dirty oil" still in there. When I changed the PSF in my wife's Honda, I felt good about doing a by the book purge - spewing the old fluid out the return hose rather than a bunch of partial suck and top off cycles as others recommend as an easier and less messy practice.
I guess it's psychological. The rational side tells me that mostly what's going on with the fluids is that the old stuff is worn, and replacing half or most is all that's needed to restore it to proper function. But there's the other side of my brain that thinks it's like washing the dishes. Even today I've seen DIY guides saying that you can judge the condition of motor oil by its color.
Of course that runs counter to many the modern fluids that run longer with longer change intervals or where they aren't specified at all. Some transmissions specify a "lifetime" fluid. However, every automatic transmission I've ever heard of leaves most of the fluid in the torque converter after a simple drain, and some owners are almost offended that isn't not at least 80% of the old fluid drained. A lot of people just do partial changes for power steering fluid because of the possibility of introducing air in the system that takes forever to bleed out. DIY coolant drains are partial. As a result, some businesses advertise changes using full-exchange equipment.
Maybe we get all warm and fuzzy about getting out as much previous fluid as possible. I probably don't need to, but when I do oil changes at home I'm not in a hurry and let it drip for up to a half hour. Even after all that the color is still a bit dark (probably about 80-90% of the oil), but I know I did what I needed. Some carmakers even specify filter changes every other oil changes, which means more "dirty oil" still in there. When I changed the PSF in my wife's Honda, I felt good about doing a by the book purge - spewing the old fluid out the return hose rather than a bunch of partial suck and top off cycles as others recommend as an easier and less messy practice.
I guess it's psychological. The rational side tells me that mostly what's going on with the fluids is that the old stuff is worn, and replacing half or most is all that's needed to restore it to proper function. But there's the other side of my brain that thinks it's like washing the dishes. Even today I've seen DIY guides saying that you can judge the condition of motor oil by its color.
