Have Guillain-Barre Syndrome. Taking Percocet. Guide

Hi, I am a 59-year-old male with Guillain-Barre Syndrome who is suffering from extreme, constant pain. I am currently taking Percocet 10-325 four times a day, however this is not helping. The main problem I am encountering is the both the strength of the medication and duration that the medication lasts for. I have spoken to my doctor about this problem, and he gave me a choice of several other pain relief medications that I can try. They are Oxycodone, Oxycontin supplemented with Percocet, and MS Contin. I am wondering which of these medications would have the longest duration of pain relief, while at the same time delivering a stronger effect than the Percocet I am currently taking. As an aside, I have had both Percocet 10-325 and Oxycodone 10 available to me at the same time due to a prescription mix up. When these two medications were taken together, they alleviated my pain completely for more than the 6 hour time period that the dose was supposed to perform for, and allowed me to take regular Percocet 10-325 for the rest of the day without any significant increase in pain. However, this solution in and of itself has not been something that has worked as well as I hoping it would work.

My question is, of the options I have been offered, keeping in mind they are the only ones available to me at this given time, I need to know not only which would be the most efficient and longest lasting, but also, mot probably the safest.

I would also like to add that during two recent visits to the hospital, each of which lasted one week, I was given 4 mg of morphine every 6 hours with no ill effects whatsoever.

Thank you for any advice you can offer.