2008 Tacoma oil in Coolant system

I have a 2008 4x4 2.7Liter with about 330,000 original clutch, but not original frame. I noticed oil in the coolant as I had been topping the oil off about every 500-1000 miles. I flushed the system hoping it was coolant particulates at first.

I drove it for another 10,000 miles and noticed the heat wasn't working great. Dealer was confident there was oil in overflow tank and said its likely a head gasket. I personally performed a block test soon after and had 0% color change. (No combustion gas is coolant).

My question is if anyone has any idea as to where the oil could bypass a seal to get to the coolant system. I read somewhere that the oil filter housing has coolant that works as an oil cooler, but i just removed my oil filter module and I can see how coolant could be introduced.

The last thing i'd like to mention is that the truck runs and drives fine. The issue currently is oil in the coolant system, no antifreeze in oil, and no combustion gas in coolant system.

Any help would be much appreciated.

 

It does sound like a bad head gasket. The test you did only tests for an exhaust leak into the coolant - one of 7 or 8 ways a head gasket can go bad. The leak can be between an oil galley and cooling passage as you appear to have.

Edit - but oil would be milky if coolant mixed with it. So, not sure what is going on. Maybe the leak is not severe enough yet to cause the milky look.

 

Are you 100% certain it's oil getting in it and not someone put oil in it by accident (or on purpose)?

It's very rare to get oil in the coolant from a headgasket leak, typically oil in the coolant is from a failed engine oil cooler or transmission cooler but you have neither being a 2.7L manual unless someone added an engine oil cooler.