Almost no difference in syn costs if you change your own. The recommendation for ST HM syn is a good one.
Huge difference in syn costs if you get it changed (I have to)-- that's why I'm using VWB. Have two $10 oil change+ fire rotation coupons left. Roughly fifteen for VWB with disposal fees, probably $60 with the coupon for synthetic. If I used syn, the ST syn hi mileage at Walmart is the best deal out there for an oil change. There was a post on here a week or so ago that it's switched diesel oils to Citgo from WPP. Wonder if the same thing will apply to other weights?
Given my HyunKia engine with GDI, I'd probably stick with a 5K OCI. On any car I've owned, I've found VWB burns down a bit over 5K, hence the reference to throwing in some HarvestKing in my sig line.