Can I use these wheels
 

I have an 81 380SL and I also have access to a set of 16" wheels from a 2000 S500. Will the wheels from the S500 fit the 380. Both have the stock size tires.

HMB, I suspect the S500 wheels would use a larger diameter lug bolt because it's a heavier automobile than the 380SL. Another important feature is the offset of the wheels. That is indicated in mm by the ET dimension stamped on the back of genuine MB wheels. Recommend calling the Parts Department of your local MB dealer and asking if the S500 wheels are compatible with your 380SL. If the lug bolts are, by chance, the same diameter verify that the S500 lug bolts are also the same length. Shorter lug bolts will lack the necessary thread engagement, whereas longer lug bolts may create interference problems with the emergency brake components inside the rear drums.

Here are some resources that can help you determine whether it will work

Wheel-Size.com :: Reference guide for car wheel and tire dimensions such as size, offset, PCD
RimsnTires
http://www.benzworld.org/forums/r-c107-sl-slc-class/1539604-wheels-will-fit-107-early-years.html

It's not an easy question to answer and the variables are myriad.

I'm running CLK wheels on mine with just a tiny bit of rubbing at full wheel turn. Spacers can be used but read up on hub centric spacers to fully understand the issues...and dangers.

The parts dept isn't going to have wheel interchange information.

You will need to check back spacing and overall width then draw things from there.

I believe others have used later wheels on an earlier car but the are pulled too far inwards. The lug diameter and spacing issue can be solved with bolt on wheel adapters if both lug diameters are called out properly