Heart disease begins with the oxidation of lipoproteins that are then taken up by macrophages, which may become foam cells, fatty streaks, necrotic lesions and then cause a heart attack or stroke if ruptured. Anything that increases the rate at which lipoproteins become oxidized will probably increase the potential for heart disease. In this study they fed 5g/day (which isn't even a megadose these days) to people with elevated triglycerides and gauged the resulting effects on the resistance that their VLDL and LDL had to oxidation. The result was a reduction in the time it took for oxidation to occur.
If it is generally accepted that PUFAs are unstable and prone to oxidation, and that you don't want your cellular membranes constructed of omega-6 fats, it stands to reason that you don't want them constructed of omega-3 fats either. Sure, triglycerides dropped, but you could do that with fasted exercise without the increased risk of lipoproteins becoming pathogenic due to oxidation and without the concomitant rise in LDL. Copper doesn't oxidize LDL or VLDL in vivo but the relative rates of oxidation should still be relevant for ROS in our blood. If 5 grams per day for 6 weeks caused that big of a change in oxidation rates, what about if you consumed that much for a year? Or if you consumed double or triple that much for a year?
It seems like the safe range for fish oil is not much more than 1 gram per day so that you're meeting the EFA requirement without flooding the system with so much of it that membranes construction is noticeably affected. I can't remember who it is in the community who recommends massive fish oil doses, but I sure as hell wouldn't do that. The best answer to past omega-6 intake isn't current omega-3 intake to "balance it out," it's current saturated fat intake to balance out PUFAs in general. Worrying about whether the oil itself is oxidized before you eat it is irrelevant if your lipoproteins are becoming oxidized as a result of PUFA-constructed membranes and PUFA transported in the VLDL as triglycerides.
Before and at the end of supplementation with 5 g/d fish oil the lag times and propagation rates of VLDL oxidation also correlated with the total number of double bonds in all PUFAs of VLDL
I pondered in another thread about whether omega 6 fats are really dangerous if you get enough omega 3s. Seeing this leads me to believe that overall PUFA intake needs to be only so large as to hit the EFA targets and not a bit more. Even better would be to take Chris Masterjohn's advice and to consume DHA and AA (instead of EPA) from normal food sources like pastured egg yolks, have them constitute a very small percentage of your diet, and avoid fish oil supplements altogether.