I have JDM Suzki Alto G 660cc, it is driven 182,000 but the mechanic recommends to out 20W 50 for the car and that is also in the car. But i feel this is isn't supposed to go for this car as this grade is used 1300cc 1000cc. I want to improve my efficiency. My current engine oil is also mineral oil, should i change to synthetic or fully synthetic and which brand?