About 2 years ago I had someone fill the hydraulic tank on my skid steer with diesel. The diesel and hydraulic tanks are in the same area, just on opposite sides of the machine... and I apparently had idiots working for me. Not sure how long it was run like that, I went to use it and found the overfilled oil tank on pretrip. I've drained the hydraulic oil 3 times and replaced the filter each time. Maybe 4-5 hours run time between. I had a fitting leak on a bush hog and noticed while cleaning it that the oil still smells like old diesel. As far as I can tell it works as it did before. Diesel as least has some lube in it. Keep spending ~$300 a go and keep draining it or?
I think the concentration should be pretty low if you've drained the system 3 times. Maybe run it 500 hours or so and then drain and change filter.
It is pretty hard to get hydraulic systems flushed out without a kidney machine of some sort. I would probably be fairly concerned about it though; Hydraulic components get real expensive quick.
I wouldn't be too concerned about it. Hydraulic systems will operate on any kind of oily mixture, and diesel has lubricity and protects against rust. Just make sure you put highly visible DIESEL and OIL labels on tanks! I have to confess, I've been caught out, I put diesel in a Mack truck hydraulic tank, many years ago. I was on a new job, the truck was unfamiliar to me, the tanks along the frame rails were all built to look the same, and even the tank caps were almost the same! - and there was a lack of decals identifying the tanks. Plus, I was probably tired and not 100% focused on the fill up, when I did it.
I'd think that the concentration of diesel in oil would be minimal at this point after 3 oil changes. I wouldn't worry too much, maybe cut your next hydraulic service interval in half if it really bugs you.
The caps were clearly labelled. Sort of suspect it was done on purpose, but that's another story.
Remember that as manufacturers try to make things more idiot-proof Darwin only develops an improved class of idiot........ One case I remember was a lube truck crew pumping grease from the central service point of a Hitachi EX3600 into the hydraulic tank. This happened more than once until action was taken to change the quick couplers to make it impossible.
If like some of the Macks at the quarry I can understand. Had more than one where the right side fuel tank was modified to be used as the oil tank for the hoist! They did have labels on them but being used as stocking truck in a quarry they were not the cleanest at times!
You'll never get it all out. You've done your diligence though. I'm thinking you're fine.
One of my R model Mack trucks is like that. It has a locking cap on it that differs from the left side tank and takes a different key that is clearly labeled, and both painted red.