Thanks to you all in the past Here's my question. I have a 2005 town car Signature L and want to change the rear axil fluid with synthetic lubricant. The shop manual says to use SAE 80W-90, but I have been told that Synthetic SAE 75W-140 may also be used. I have never changed the lubricant before and the car now has 204,000 miles on it. Is it okay to use the SAE 75W-140 synthetic or must I use SAE 80W-90? Thanks to all.