Technically the choice of oil should be SG or SH grade. And good news is that still such oils are plentifully available in Pak. market.
However, we've done experiment on many cars who reduces oils and we fed them PTT Performa 20/50. Till yet 99.9% results are satisfactory. Even we done it on Mercerdes 2.8L (Early ninetees model) and it stopped reducing oil.
OK for this time, go for PTT Performa 20/50. Its from Thailand and fully available in market.
There are two grades available in it. SM & SN.
Closely check & give your preference to "SM" grade and pour it. In worst case, if SM grade is not available, then try "SN" grade.
Price of this oil is exactly same as of Zic Hiflow 20/50, Havoline 20/50, or Shell HX3 20/50 or anyother available oil in 20/50. But performance of PTT Performa 20/50 is relevantly better than other oils.
Closely check, if car stopped reducing oil, then keep the change interval as 3K for this oil.