Just use mobil, castrol or any other reputed brand in the market. If the car severely burns oil, then switch to thicker oil. But for normal driving 15-40W(I think the owners manual recommends 5W-30W) Mobil/castrol,etc oil should do just fine. If the oil level goes down a bit just top it up. I use 5W-30W fully syn. oil on mine with 207,000km and it consumes a bit more than 1/2 litre of oil between 10K km service intervals. But given the mileage that you two have on your cars, i seriously doubt that you would have any issues.