All of the advantages (of which there are many that are real) of synth are lost when it is mixed with dino. This includes pre-packaged blends and homegrown mixes.
Off-the-shelf additives are to be avoided for the most part. I used to be a Slick 50 believer, but I have been enlightened.
There's a lot of misinformation, myth, and just plain lies flying around out there, especially from the automotive service and dino oil industry. They want to keep you stupid, and preach the mantra of dino and 3,000 mile change intervals.
Be an informed consumer. Check out
http://www.autoeducation.com/autoshop101/oil-change.htm and practice your reading comprehension skills.
I switched to Mobil 1 Synth at 50k, I use a foam open-element air filter from ITG, and ditched Fram for Motorcraft in the oil filter department.
