The flush will make you pretty acidic, which I find to be a dangerous practice. The stomach can burn for hours after ingesting the mixture of oil and lemon juice. This will drain your body of alkaline mineral reserves. Additionally the purging of bile salts will also drain you of alkaline reserves. That's a double whammy. This *will* drag your pH down and you can measure it yourself with simple litmus paper (saliva test.)
The stomach buffers acid foods by producing sodium bicarbonate. If you are running low on alkaline reserves your stomach may not be able to buffer a large dose of acid all at once. After swallowing the oil/lemon mixture take a jar of 8 ounces of water to bed with you, having mixed 1 tsp of baking soda (sodium bicarbonate) in it. Sip this over the next 20 minutes and this will totally change your experience. You won't have a burning stomach, and the mixture will drain much quicker into the small intestine.
I have done this and it in NO WAY interferes with the flush process. In fact, unless it was a coincidence, this improved the experience by 100% for me, and I barely had any discomfort. I got LOTS of stones the next day, so I consider this to be part of the protocol for me now. I did a lot less tossing and turning that night - hardly any. I did not feel punk the next day and that is a first for me. I felt better than I did the day before. Why do we usually not feel better the day after a flush? I believe because it makes us acidic, and being over acid is the basis of pain and disease, including cancer.
Check out the Ask Moreless forum if you want to know more about how to get our pH into a safe range in a hurry. He has an alkalizing drink that is quite helpful. It's cheap as dirt too - made with common materials that you can buy locally (or on line.)