Actually, putting oil in the PS reservoir just makes the steering harder, but I've seen it done lots on the big trucks!!!! Same tight steering required, especially when not moving. The fact that it was black when he first opened it up, however, worries me. It should not be that dirty/burned. Good time to flush the power steering system and put in fresh "Power Steering Fluid," or with ATF, as mentioned earlier. Regular oil has too high a thickness, resulting in that hard jerky steering . . . not to mention how big your biceps will develop, if you leave that in. Generally, oil does not hurt the power steering pumps, and many truck/fleet owners use oil in their truck PS systems when they are getting old and starting to leak a tad bit.

Actually, it is highly unlikely that you would find 15-40 that is not rated for diesel at a place that does oil changes, or the local auto parts store. I've yet to see non-rated 15-40. I believe 15-40 is a designation, only for diesel rated oil. However, I would worry about any of the "pink" fluid winding up in the engine. In my motor pool, we used ATF to clean out engines, right before we drained the oil. If left in, it can eat the seals, as it has the highest amount of detergent of any of the oil based fluids.

If I weren't worried about cross contamination from ATF in the oil, I would say, change the oil filter, and top off, and be done with the oil portion, until your next oil change.. Obviously, he didn't change the oil filter, if he didn't know where it goes. Also, he probably used Shell Rotella . . . the new bulk oil for diesels. Personally, I hate Rotella, . . . it's a dirty oil that will leave a lot of gunk/grime and metal shavings clinging to the inside walls of your engine (not enough detergent). Especially if you have long climbs on hills, or drive a lot in the summer heat with the AC on, I would use Delo 400 . . . a thick oil that doesn't thin out under heat like other oils do. Castrol Tection is known for suspending dirt from the engine walls into the oil, so the filter will remove it. Castrol is a very good choice. Mobil Delvac is also a good blended oil, with an excellent reputation. You can buy all of these oils at Auto Zone, or Advance Auto in 1 gal. jugs. I have also used the generic 15-40 under the Walmart label (Tech Line), which is the same oil that you find at Tractor Supply. It is a good cheap alternative to the brand names. Now there's also "Blue" by Valvoline . . . so, you have many choices. You may also want to put Lucas oil stabilzer in it if it will be working in hot conditions. Personally, I would not want to switch back and forth from conventional diesel 15-40 to Synthetic and back every season change. Not a good idea. Find what you like, and stick with it.

And, welcome to the work of MBZ diesels . . . hope you're enjoying the ride.

__________________
1983 300D, the "Avocado"
1976 240D, 4-spd the "Pumpkin", SOLD to Pierre
1984 190D, 2.2L, 5-spd, my intro to MBZ diesels, crashed into in 2002