For your cultus it is possible to use 5w-30, 10w-30, 10w-40 and some others as per manual. Generally 10w-30 and 10w-40 is cheaper, but you have to change more often as it is semi-synthetic.
To answer your Question, below are affordable fully synth 5w-30 oils.
1. PSO Carient 5w-30 Fully Synthetic (API SN)
2. PSO Carient S-Pro 5W-30 Fully Synthetic (API SP)
3. Shell Helix HX7 plus 5W30 Fully synthetic (API SP)
4. Total Quartz 9000 5W30 Fully Synthetic
I would personally recommend to go for the PSO one as 1st choice as its the cheapest and best value for money, and good enough for cultus, especially since you want to go on a budget. Plus its also good to support local brand.
Caltex is also good but more expensive than the above 4 mentioned.