Will diesel oil damage oil rings and seals on a petrol motor?

You mean "motor oil for diesel engines," rather than "diesel oil the fuel." And the answer is simple: No. It will not. All the popular and not-so-popular brands of diesel engine oil sold in the US carry two ratings: a "diesel engine rating" and a "gasoline engine rating." They do this because most trucking companies also own some gasoline engine vehicles, like pickup trucks for driving to the truck dealership on parts runs, and an oil rated for both gas and diesel engines allows them to put the same oil in all their vehicles.