Afternoon
I think I must be missing something here....
How is it you've come to the conclusion there is actual engine oil in the coolant?
From what can see above, it's because you've seen an oily looking film on top of the coolant and because the coolant doesn't look spotlessly clean?
As has been mentioned above, the cooling system operates at high pressure, so would force coolant into your oil if a gasket or seal was gone - and if that was happening, you'd be seeling a LOT of emulsified oil, and/or a drop in coolant level and increase in oil level in the sight glass.
I think you're worrying over nothing personally, coolant always gets a bit skanky over time (hence flush and renew every once in a while is a good idea). Coolant reservoirs also get mucky (no idea why, but they do).