Hi Pradeep8,
Take a look at this guide. This covers most of the basic components that will cause this issue. It very well could be the cooler as when these fail, they allow the fluid to mix.
https://www.2carpros.com/articles/coolantantifreeze-in-the-engine-oil
If the compression is the issue, here is a video that explains very well how to tell.
https://youtu.be/5zpNjYmmiYY
If they checked it out, I would assume they eliminated a blown head gasket and if so, the cooler is the next most likely cause.
If they are guessing at the cooler, I would definitely have them or someone go through these other items mentioned in the link above and video.
Please let me know if you have questions and I will be happy to assist further.
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Sunday, June 2nd, 2019 AT 4:59 PM