About Engine flush, lets put this fluid aside. Because its not v. important to use it unless certain situations.
For example if someone bought really bad 2 number oil un-intentionally and use in the car considering it genuine upto few K. And after when he knows that he is using really v. bad quality engine oil, so in such cases using Engine Flush is recommended.
Engine flush is actually 0/20 engine oil which has low flash point of ~60'C and perform good cleaning when mixing with engine oil whose change is due.
Shell 5/30 fully synthetic is good fuel saving oil.
But mixing it with ZIC A+ 5/20 is NOT GOOD idea.
No. 1. If Shell 5/30 was not available at one shop, then you should search the same oil at other shops as priority.
No. 2. If need to top up with same synthetic oil (which was not available), then any good pure mineral oil was fine. Like 10/30, 10/40 or 20/50. Mixing synthetic oil with synthetic oil is not good.
I'll suggest to run only max. 2K with the oil mixture you've already made in the engine.
And next time, better to buy 4L oil and keep the balance well saved in tightly closed bottle at room temperature (20~25 Degree C) & keeping it away from extreme conditions like direct sunlight, close to kitchen, close to bathroom, etc.