I have a 1999 F-150 4x4 5.4L and i live in hot florida. Reason why im changing my fluids is because i dont know when it was changed last, also i have a vibration around turns from the LSD, which is why i bought the motorcraft LSD friction modifier additive.

In terms of what fluid to use what should i use. I currently have 4 Bottles Of Valvoline 80w90 Conventional. Should i go to synthetic? also do i have the wrong weight, etc?

What should i use. I want to get this fixed asap so i don't prolong possible damage.