Mad Gypsy

Since I can't find anywhere local that sells Castrol RS 10/60 (currently in the car) Would it be ok to top up with Mobil 1 15/50?

scoobyslut

I would be reluctant to mix "viscosities " of oil unless it is "desperate" to do so. I would only do this if it means running the engine on the low level bar. if its simply a "top up", then suggest you obtain the same oil, unless you are about to do an oil change anyway, where such a varience will not really matter.
I am surprised it it actuall "using" oil, I hope you are not topping up to the maximum level mark, if so, you engine is draing it out anyway, make sure you fill to correct level on dipstick,this may be a reason as to "why" it keeps dropping.