Thanks for the reply bro.
I went ahead and re-read all the old threads about this topic as well and these are the three most recommended oils:
Shell Helix ( yellow daba )
#1 It seems that Shell and ZIC are coming in bad quality nowadays and hard to find genuine oils of these brands, is that the only reason you guys are recommending " Caltex Havoline " nowadays???
Famous brands like Zic, Havoline are coming fake in abundance. If you can source out original oil from reputable/reliable source then go for ZIC or Havoline whichever you find original. Apart from that Total 20w50 is good too. (All are good but only if original). But on my recommendation try Petromin Ultra once 20w50. I can tell you reliable source for Petromin.
#2 Should I use Caltex Havoline of " 20w50 " grade or something different?
Answered above. By the way the owner's manual suggests 10w40. If your engine is in good condition you can opt for it.
#3 Which filter should I get, will the guard filter work good enough for 5000 KM with this oil, else recommend me a couple of alternatives.
If you can find Leppon it'd be really good else Guard will do the job.
#4 When oil is poured on the filter, should I just put the remainder of the oil in the engine oil or should I pour 3 litres more in engine oil???
You have to put just enough to lubricate the filter and rest will be poured in engine.
5 I've read many people recommending to change oil after 2500-3000 KM in these old cars, but you are recommending 5000 KM, will the oil be good enough for that long???
5000 KM is okay, 2500-3000 are pre-mature oil changes
#6 Lastly, should I get an engine flush of this old mehran in hopes for cleaner engine and better performance, or will that have a negative effect resulting in more leakage???
It is a really good product. You should go for it. It helps in clearing the deposits from engine.
Thanks in advance!