The net effect of adding 1/2 or 1 gt of anything
in your 4.5 gt really doesn't change it much
as you can check in the shell viscosity mixer as its only changing it by 1/5 anyway. I do use add oil to aim the grade to where I want to try.
On my daughter's oil consuming tercel, I add 1 qt of 20-50 Castrol HM to see if thicker helps stop her consumption and rear main leak. On mine I add some moly laden VSOT or 10-40 to see if moly or thicker helps. Its like a low cost preview if you are going to change in 1k miles anyway.
On yours, you will change soon if you stay with 3K OIC so adding a blend with maybe 20% syn is like adding 5% syn. If you want to experiment and see how a blend works with GM dino I'd add all syn so at least you get a blend for the entire 4.5 gts. Costwise I'd add any dino since you'll drop it in a month.