Sir, as a permanent solution, you have to overhaul the engine.
And as a temporary solution, you can use 20W-50 SG, SH or SL grade and pour 1 tin of STP (Oil Treatment). This STP price is like 230/400ml. Cultas takes 3.5L engine oil with filter. So you can use 3L oil and 1 stp.
The summer is almost started and it will be OK. In the morning, it may take 1~2 sulfs extra, but this formula is used for all those engines who are dying because of weak rings/piston. This STP is also used for Noisy engines and generators.