Since mehran uses a traditional flat tappet engine, it demands high zddp levels which are witnessed uptil api sj only. Api sL, sm and sn lack zddp which can result in a damage to ur ride. I would recommend a proper format to follow. For a lighter engine and good fuel economy u may start from sgo premium 10w40 api sg, available at all suzuki dealerships. It works upto 3000kms but since u mentioned that the previous oil is dirty, drain sgo around 2000kms. Be cautios not to buy sgo ultimate which has the same 10w40 viscosity but different apI grade. But if you witness some reduction in oil then u may go for a thicker oil. The next oil should be zic m 15w40 api sj. Use it only if not satisfied with sgo. If zic m is also being reduced then shift to any 20w50 oil, preferably caltex havoline. But never go for zic hiflo.