White foam in A/C...
A white foamy look in the sight glass can mean several things - for instance it could mean that water has entered the system and is turning the mineral oil a milky color, a very bad situation indeed. It could also mean that the system has been converted to R-134 and the original oil wasn't completely removed, and the ester oil has mixed with it to make a milky looking lubricant, a not desireable but usually not fatal problem, at least for awhile. A good conversion process requires removing the oil from the compressor and flushing the components to remove any remaining mineral oil, which is not compatible with the ester oil used with R-134, regardless of what the 39.95 box says on the side. Since you didn't describe the refrigerant and history of the A/C, we cannot say what your situation might be.
'82 300D/4.3L V6