You know I showed you how to average numbers when you did a similar mix, right? Add both of the low numbers and divide by two. It's a 10w on the low end if all else is equal.
For percentage change: Find the difference between the beginning number and the ending number, then divide by the beginning number. Then multiply by 100. Going from up 5w to 10w is a 100% increase. If going down from 15w to 10w, that's a 33% decrease. Respectfully, those are not hitting the target of essentially T5 15w40. With respect to a semi synthetic blend yes, in terms of potential oil grade if all else is equal, it's not.