salaam all:
brother just follow the car owner's manual for service intervals and recommended oil specs.
back in 1979 i told a guy at the service station at PSO (F-8/1) to correct the differential oil level of my toyota sprinter. on following the chap he went to the back-yard and was filling the oil-syringe with spent / used oil collected from various vehicles in a drum. on enquiring he insisted that it was the right kind of oil they put in, calling it "c-oil"..... allah janay kia matlab hai 'c-oil' ka???
anyway from that incident till date i replace all the lubricants myself.
i suggest EP-90 / GL-5 gear-oil for manual transmission and final drives for cars.
you can trust "Caltex-Thuban" and "US Petroleum" brand name with the same EP-90 / GL-5 rating.
remember never trust what most of the local mechanics say as they have very limited technical know how. go by-the-book, and keep changing the transmission / differential oil. you will have your gear shifting like 'spoon in honey'! & incase you have a seperate final-drive (Differential), that too will remain slick.
regards