I was advised (relative to my other car, a Peugeot 405) that oil in the cooling system could possibly be indicative of a cylinder head gasket failure (in my case there is also the possibility of a defect in the oil cooler : doesn't apply here, obviously).
A cylinder head gasket problem might be introducing a bit of excess pressure into the coolant system which would increase the chance of failure elsewhere (e.g. in the rocker cover gasket or water pump gasket as previously discussed), without necessarily being strong enough to blow hoses.
I understand that, these days, it is easy to diagnose a head-gasket problem by introducing a special dye into the coolant which changes colour in a specific way if it encounters cylinder combustion by-products.
GTLs are notoriously prone to head gasket problems as the cylinder head retaining bolts are barely adequate (as regards length). There have been umpteen posts about this on the Forum over the years.