Had to have a leaky rear axle seal replaced while on the road and was able to get it done right away on a weekend at a national "tire" store. They had to replace differential fluid as part of the job and used 75W-140 full synthetic with friction modifier (for my limited slip). Owners manual specifies 75W-90 full synthetic with friction modifier so is there any problem with the 75W-140 they used, and if there is what is the problem?

 

75w-140 synthetic is the correct. The 2004's were originally spec'd for 75w-90, but were later back-spec'd for the 75w-140.