this is what i was told, if your car is over 100K miles, should be best if stick with regular:

1/. there might be cracks in your engine but covered by the dirt leftover by regular oil hat's what keep ur engine in okay condition

2/. if you use synthetic, best if you flush your engine before use them, therefore, will flush away the dirt build up in those cracks and yet make your situation worse, it'll leak and stuff.

so Synthetic Blends is the 2nd Option where it won't make much diff. to Very High milage & old engines.

Full Synthetic brands such as AMSOil are NOT reccommended, eventhough It only require you to change oil FOUR time / yr or every 25,000 Miles.

that's what I read in the report + review. I can't put it well enough so hopefully you can understand it.