I have a magnesium case 1977 2.7 ltre and i use a high grade lubricant spray called "Inox" its an oil based and i use it to clean all my engine and gearbox and stays on for months..its anti corrosive and it protects the engine casing it gives the metal a bit of a polish look as well.
https://www.inoxmx.com/products/mx3-lubricant/