Do you tow with your V6.
There are thousands of 4Runner owners on the road that have V6 and have never heard of this forum. When I worked for Toyota we had 2 (04 and 06) V6 runners with "head gaskets" problem is before I left the dealer, one had come back and according to the tech, the block was warped and it needed a new motor. He said he suspected it right from the start, because he said these head gaskets do not just blow, something had to have given enough to cause the failure.
THIS is just an FYI. I do not have a v6 and I have never worked on one, I just wanted to let you know that from what I have been told at Toyota, the aluminum V6 blocks can warp under heavy towing and temps.
I may be way off here, I am just sharing, what little I have heard. Good luck man