I planing on changing the gear oil in my Moms 93 crown victoria with 135,000 miles on it, and my choices are Royal Purplr (syn) , Mobil 1 (syn) and Valvoline (dino). I'd like to use a syn oil and change it every two years. Thanks for any help
I seem to remember something about corrosion in an AMSOIL White Paper regarding Royal Purple. But, I have used all of those brands successfully in many different cars including racing applications. I say whatever you can find on sale, all three are good choices.
Personally when i am out of Redline or Amsoil gear oils I use Mobil 1. I believe it is a very high quality gear oil. Have used and abused it in many jeeps and full size trucks in off road applications with outstanding results. Its also readily available which is nice.
My gosh, she's only averaging 9k miles a year or less. And I really doubt she's hard on the vehicle. Also, there's the excellent reputation of the 8.8" rear (the car; not your mom).
Syn gear oil and 2 year OCIs are a waste of money in this circumstance. Heck, dino gear lube and 2 year OCI's are a waste ...
Put in some Mobil 1, and forget about it for the next 10 years.
My 96 CVPI had a minimum of 5 years and 45K miles on the fluid in the differential when I changed it. The gears look absolutely brand new, and there wasn't a single shaving of metal to be found, and I drive that car like I stole it.
Use anything with the proper specs and the differential will be well protected.
I used Amsoil severe gear fluid 75w-110 with fine results in my Grand Marquis. Can't speak to the use of the other products you posted. But I do recall some negativity towards the Mobil 1 gear oil. A search may lend you the info you seek. As a side comment I would discourage you from changing out the synthetic fluid in such a short time as two years. Unless you totally hammer your vehicle and drive it on dirt roads and in deep water....well, you're wasting your money. I would call it over-kill and try and leave it in at least 3 to 4 years (and that's if you drive it 15k miles a year).
Just my opinion.