Yes Durablend is part synthetic. However, you could add full synthetic, or better yet if you are nearing the end of the OCI, just add conventional as the syn won't have much benefit in say the last 500 to 1000 miles of an OCI.
whay just no to add semi syntetic as this is what's now inside the engine?
Because if you're near the end of your OCI adding a semi or full synthetic would be a waste of money. Since you already have semi in your car, adding a little dino oil won't hurt a thing, and will save you a little money.
Another option is to let the level go low towards the end of the OCI. You should have a safe range that typically represents a quart, nothing wrong with being half quart low, so long as you're in the safe range, by the end of the OCI.
I should add one thing re Durablend. It is semi syn in America. I assume the same for Durablend sold in Israel but not necessarily true.