i changed out my frint an drear diff fluid with royal purple last month, i noticed about a 1 mpg increase, Note: if you have the skid plat on you either need to remove it to replace the front or get a lube pump. i recommend the lube pump, it's only about $10.00 and makes it a heck of a lot easier.
The MDS system deactivates the valve lifters. This keeps the valves in four cylinders closed, and there is no combustion. In addition to stopping combustion, energy is not lost by pumping air through these cylinders. For proper operation, 5w-20 oil must be used in engines with the MDS feature. Failure to do so may result in improper operation of the Multiple Displacement System. With that said any type of 5w-20 oil will work and using a good synthetic will provide a higher film strength, reduced friction, better corrosion protection and higher fuel mileage.
It's my wife'd XK, she wants to run dino for the warranty, a tech at Jeep told her that synth doesn't really help. I run mobil full synth in my 03 Liberty and our 02 GC. She will come around sooner or later! O yea, we have the 4.7 which runs 10w30.
I think it is safe to use synthetics everywhere except for the transmission. I think there is one company that make the atf+5 in synthetics but i still am not 100% sure that i would trust using this in the very touchy chyrsler transmissions. for that i would go to the dealer and get the mopar brand and make sure you change every 30,000 miles.
I have used Moble 1, 5w30 from the first oil change in every jeep (and two Dodge Dakota trucks) we have had since 2001. All were the 4.7 liter engine. I have never had a problem. These are great engines. I also use either the K&N or Mobile 1 oil filters. I believe that this is a good set-up and highly recommend it.
I will be changing the new XK as soon as it reaches the 3,000 mile threshold.
I hadn't really thought about using synthetics in the differentials. I will have to look into that.
I have switched to AMSoil synthetics motor oil https://www.amsoil.com/ on my 96 VW Cabrio 70K miles and my 95 Toyota Camry 145K miles in the last year. Two years ago I switched from regular oil to Castrol synthetic. AMSoil is a great oil and allows fewer oil changes. I have only let the oil go to 9000 miles though. Stated by AMsoil is 25K miles or 1 yr which ever comes first. They recommend changing the oil filter every 3K miles which only requires an additional pint of oil added lost by removing the oil filter. They suggest using their oil filters or WIX filters which they supply both. Even at 9K miles the oil was still slick as hell and amber in color. I even took samples and sent the samples in for analysis through my AMSoil dealer and the results came back very good. The Toy Camry's are known for sludge from the engines and even the oil at 9K from that car was slick and amber! I have used quakerstate, mobil, amongst others in previous cars as well as these and at 3K miles the oil is black and heavy (not too slick). I am religious about changing the oil every 3K miles here in dusty Arizona and was EXTREMELY sceptical at first about letting the oil go 25K and still am. But this oil is slick! So slick that the rpm's on my VW went up 150rpm after putting it in that I had to dial the rpms back down. My AMSoil dealer said this might happen. The cars pur now better than ever before and the V6 camry with 145K miles on it sounds like a new engine and when in idle is hard to recognize that it is even running. One of the guys at work restores old muscle cars and turned me onto the stuff. He has a Dodge Ram 2500 and just recently put the AMSoil transmission oil in his truck. He said that the transmission had never moved so smoothly before even compared to the day he pulled the truck off the lot. I am still taking oil samples every 3K miles when I replace the oil filter and clean or replace the air filters (WIX) and am sending them to AMSoil for analysis. They probably have my file framed on the wall by now but it keeps my anal engineer mind at ease and my cars have never run better. I plan on putting AMSoil in the Jeep at the first oil change (900miles away). I think I need to contact my AMSoil dealer right now and place an order!