Before my 83 240 turbo was wreaked I was planning to do something similar. I switched it syn oil, but the engine inside was dirty with old dino oil curd. I did flush with amsoil motor flush, but did not see my internals sparkle. Some places have an engine flush machine that I hear is good but goes about a 100bucks.

On the otherhand, my 89 which I swicted to mobil 1 at 170K miles, has a clean head after 75K miles of syn oil changed every 5k miles. All internals parts are clean and all of the crud and deposits are gone. You can see the AL! The oil stays clean after 5k miles. So yes, syn oil does a good of cleaning.

Change oil after you cleaning method with a cheap dino brand and then switch to Syn oil to make everything is clean.