Hi, Due to increased oil consumption I'm looking to change the oil in the wife's 2003 Smart car. From a Google search I'm led to believe that a 10W-40 fully synth may help, currently I use 5W-40 fully synth. Though I appreciate that an engine rebuild may be on the cards before too long. Halfords sell, Mobil Super S 10W-40 semi synth GSF sell, Titan Diesel 10W-40 fully synth 1. Since I change the oil every 5k is semi synth OK? 2. Are oils branded for diesels OK for petrols? Thanks for any advice.