My 2007 Tacoma loses about 3.5 qt of oil out of 5 every about 1500 mi. I know it's low because the engine starts periodically rattling, I don't know which part but it's 100% correlated to the low oil level. I know the measure because I drained the existing oil in a bucket to see how much was gone. When I add oil, the rattling stops. The remaining oil didn't look particularly dirty:
This is the second time this has happened since I replaced my head gaskets last September so I'm worried if I didn't seal it right (as there might have been minor warpage on one of the V6 deck sides but very minimal just outside the factory advised threshold, which I guess is conservative) so I may be leaking oil into one or more combustion chambers.
The first time I thought it may be a leaky oil pan gasket so I replaced it (without much analysis because it was easy and the part was cheap). But it happened again. What I'm hoping is that the oil pan may be warped so it still leaks even with the new gasket. Because, in both cases, the amount of oil that was left corresponded roughly to the size of the oil pan.
I did check around the engine and everything looks pristine, no visible leaks on around the timing chain cover or cylinder head covers. Also no signs of leakage around the oil pan but then again it may have dried after it leaked down to where the joint is.
Considering the above symptoms and metrics, should I start suspecting the I'm losing it due to a cylinder leak and should I perform a cylinder leak down test, for which I have all the tools (though have never done before)?
UPDATE 1: Every single spark plug looks like this
UPDATE 2: I just did cylind er leak down tests on all 6. Pumped the left gauge to 90 psi on each cylinder, before making sure it was in TDC. The right one was between 75 and 80 psi on 5/6 and only one cylinder wouldn't go past about 65 psi.
Do these test results indicate a problem? Should the right gauge always be the same as the left one on every cylinder?