I'm surprised to hear this. I have 2 xfires and haven't noticed any oil loss whatsoever with Pennzoil Platinum Euro. I just changed their oil today and neither seemed to lose even a drop. Also, although the book says xfires take 8.5 qts, in reality they take 8 and will be overfilled with 8.5, but I assume you already know this. I change the oil through the dip stick tube with an electric oil pump which makes it very easy. What miniscule amount of oil remains, and it must be very little because I remove 8qts which is what I put in, doesn't really matter since, as said, I put in 8qts of fresh oil. Pennzoil Platinum Euro 0w-40 is harder to find, but the 5W-40 and 5W-30 are easier to find and a little less expensive. The Quaker State versions are more common and are essentially the same oil and a bit less expensive than the Pennzoil. I just paid $120 for three 5qt jugs of the Pennzoil Platinum Euro through Amazon, and it can also be found occassionally on Ebay for less, though sometimes sold by some individual overseas who apparently just takes your order and then reorders it from someone else which ends up being out of stock and then requires a refund.
Here is the link for the oil pump I use: Here is the jug I use for the removed old oil: The local auto parts store (O'Reilly's) has a free used oil dump.