Mazda's DI engines aren't as prone to intake valve deposit buildup as some (early VW DI 2 litres, for example), but it still happens, and can still be reduced by using an appropriate oil. A Google search for "VW 504.00 oil" will show lists of approved oils (note, they're ALL 5W-30 or 0W-30 - huh, how about that, water thin oils vapourize more ...