Hi all

I bought about 3 weeks ago a 2004 Nissan almera 1.6i as a new family hack.

It currently has 63000km on the clock and a full service history every 15000km.
The last two services it was serviced with Mobil 1 which as far as I know is a synthetic oil.
My questions:
1. The previous services was mineral oils. Mineral oil and synthetic oil don't mix very well. Could this have caused problems.Sludge maybe?
2. I am struggling to get Mobil 1 and want to go back to 15w 40 mineral oil.
Can this be done? What procedure can I follow to do it.

Regards