According to my understandings, this is not good sign.
ZIC A+ is 5/20 and most thinnest oil and very much towards fuel economy. This is good synthetic oil in deed. But cannot use this oil in may cars like Mehran, alto, bolan, cultus, etc.
Shell Ultra blue one is 10/40. Its quite more viscos than zic A+. So engine will feel happy with this thicker oil, sound will be less, and pick up will increase. But this oil (10/40) is “not” synthetic oil, and also not gives better fuel economy either.
Also there can be another valid reason that your car doesn’t actually required to feed 5/20 oil (zic a+) but infact you kept feeding it. But now when you feel 10/40, then engine feel good, then it means that engine doesn’t require this thin oil. They actually need thicker oil.
Which car you have, mileage, driven, change interval of oil & filters?
