1999 Ford F-250 oil in water

There are a number of things that can cause this, but we need to start with these couple of guides because the most likely cause is the head gasket has failed.

Take a look at these guides and let us know what you find with this:

https://www.2carpros.com/articles/coolantantifreeze-in-the-engine-oil

https://www.2carpros.com/articles/head-gasket-blown-test

Basically, if it were me, I would jump to checking the block with the chemical block testing that is shown in that guide above.

Once we know this info, we can determine the next steps. I suspect the engine overheated at some point and if that is the case then I am pretty sure you have a head gasket issue.

Please run through this and let us know what you find. Thanks