I have a 1968 Chevy Impala with a 307 and a Powerglide. Recently the transmission started slipping so I changed the oil. When I removed the drain plug thick black oil came out. So at this point I wondered does transmission oil become tick and black after 37 years? or were the previous owners stupid enough to put motor oil in the transmission?

After I changed the oil and the filter the slipping improved but did not go away. So now I'm wondering is the transmission shot? or is the problem something simple like tightening the band?

If the problem is a band adjustment how do I adjust it and what tools do I need?