1987 300TD OM603 lifter noise...285,000 miles, will synthetic help? BTW, I live in Jacksonville, FL

Looking at doing the following to help with lifter noise in lieu of cracking open and replacing the lifters:

it currently has Mobil non synthetic 15w40

Thinking of adding Marvel Mystery oil and/or turpentine or ATF to the crankcase and idle in the driveway for an hour or so or taking it for a 50 mile drive or so and seeing if it makes a difference. Any suggestions on additives to the crankcase or length to idle or drive. I will do an oil and filter swap after this procedure...it's due anyways.

As for oil Amazon has 0W40 diesel rated for $26 for 5 quart jugs on special 5W40 is around $40. Any thoughts on the oil? One of my buddies recommends Rotella.

I am also going to do a diesel purge. Will I want to do the purge before the oil change? Will the purge contaminate the oil?