can a mechanic tell if the last oil used is mineral or synthetic from a dipstick

What is your question?

On 8/27/2019, I specifically said to use 1 qt. of 20/50 & 4 qts. of 10/30 to change the oil. Have been using for the past 5-6 yrs. Car was blowing lots of smoke, then quit after using the formula above. Engine has over 250K miles, original engine. This last oil change blew a huge puff of smoke & continues to blow every so often. I want to add more oil, but concerned they put synthetic in & I use the oil mix, NOT synthetic. Can one determine what type of oil has been used by looking at the dipstick? What should I do?

What makes this problem better or worse?

after it heats up

How long has your 1994 Ford Thunderbird had this problem?

since 8/26/2019