15W50 is very thick when cold. Will take ages for the tappets and turbocharger to get sufficient lubrication.

5W50 is just as good at hot temperatures but a lot thinner when cold.

I don't know why anyone would use 15W, apart from in a race car with hardly any cold starts or in a bush taxi...


Anyways, yeah the acid thing is exactly what I'm thinking. I might just end up filling in some cheap oil for the storage and then changing it again next year.