The issues which can arise when switching brand and/or type of hydraulic oil, is a question I get a lot. And the crux of this problem, is:
1. You can never completely drain ALL of the original oil out of the hydraulic system; and therefore:
2. There WILL be some mixing of the original and the new oil. So how do you make sure they are miscible (compatible)?
Oil additive makers and oil blenders face the very same issue when they want to run field trials to compare the performance of their latest creation with an existing 'control' fluid. And the way these guys approach this problem offers some insight for us regular hydraulic hacks.
In one field trial I read about, the 'new' oil was to be tested on several excavators and wheel loaders of different sizes - across five different work sites. The oil change procedure was: drain the machine's hydraulic tank, refill with the new oil, operate the machine for 10 to 15 minutes and repeat - until analysis indicated there was less than 5% of the original oil remaining in the system.
In practice, what they found was: it took a minimum of four complete reservoir changes to get the concentration level of the original oil down to 5% or less!
1. Obviously, if it's a big reservoir, four changes is a LOT of oil (to waste); and
2. The new oil still has to be compatible with the original--even at 95/5 mix ratio.
So here's what our intrepid oil testers did by way of a 'compatibility study': Mix the new and original oil in ratios of: 100:0, 75:25, 50:50, 25:75, and 0:100. The mixtures were then stored at: -18°C, 0°C, 20°C and 65°C for one month. Examination for precipitates or other changes where made weekly (in this particular study, no incompatibilities were found).
The rigors of this approach to switching oils may not be appropriate for the average hydraulic equipment user. But the insight is still instructive. With only a single reservoir change, the mix ratio of the old and the new oil is likely to be quite high--perhaps as high as 25:75, and possible higher depending on the system. Which means at the very least, a rudimentary 'compatibility study' (mix, shake, wait and observe) is a very good idea.
In fact, not doing this relatively simple test could be a very costly mistake. And to discover six other costly mistakes you want to be sure to avoid with your hydraulic equipment, get "Six Costly Mistakes Most Hydraulics Users Make... And How You Can Avoid Them!" available for FREE download here.