I have a 97 F150 with over 77k miles,3:55 open face rear.The tag says "75w140 Synthetic Only" on the rear end.Do i "have" to go back with synthetic or can i go conventional? And do i have to use 75w140 or a different weight? Any advice is greatly appreciated.

 

Hi,
Please do yourself a favor - put in what it says.
It is expensive but it is for 100000 mi and you would not have problems.
Search the forum to get more info.
Alex.

 

Just for your information I have an 01 4.2 5-speed with the 3.55LS. I just bought some Royal Purple 75W-140 synthetic on-line for $7.36 a qt. My rear-end calls for 5.5 or 5.7pts so I bought 4 qts to have 1 qt as a spare. I have read alot of good things on here about Royal Purple, along with Mobile 1, Amsoil and Redline. Redline does not have 75W-140 so some guys use their 75W-90 but I wanted to go with what was recommended (75W-140).

The Amsoil is pretty expensive per qt. I think it was something like $11-$12. I would have went with Mobile 1 hoping to buy it here somewhere local but no luck only 75W-90 in the mobile 1.

I also bought the Royal Purple Max synthetic ATF for my manual transmission at a cost of $6.99 a qt. Bought it through northerautoparts.com

I plan on changing the trasmission and rear-end fluid every 30,000 miles.

 

VERY IMPORTANT...it protects the clutch linings in the limited slip gearing...i also have a 97 with 3.55 and 80000 started this wierd whinning at above 40 MPH,just kept getting louder.took it in yada yada yada. 600 bucks and all the bearings later(pinion,carrier and axle) plus seals. sounds good and a good thing i did take it in ...life expectacy was short and could have cost three times more if it had tanked

 

TexfordD:

You made a good and important point. I forgot to mention that in my post so glad that you did. As a matter of fact I asked (e-mailed) Royal Purple and asked them if I need to add the friction modifier and there answer was no, that it was already in their 75W-140.

I then decided to ask here on this site and actually posted my question to Mike in the Superchips forum. He said what "he" does is ALWAYS adds the friction modifier regardless if its already in the brand you may be using be it Amsoil, Mobile 1, Royal Purple, Redline etc. So after getting that reply from Mike who does alot of high performance testing on the F150 I decided that I would add the friction modifier to the Royal Purple when I do get it. It cost like $5.50 from my Ford dealer for 4oz. which is what I need for my rear-end.

 

thats why i caps the very important....nothing like finding out when your trucks up on the lift....of course the guys at the axle shop said mine was a fluke(lucky me)...by the way thats where i took mine...to a shop that does nothing but axles and driveshafts...no tune ups,no tires...nothing but axles...i fiqure i saved about 250 bucks by doing that...thier faster,know what thier doing and don't give you a case of the "maybe's"...that "maybe" cost you a flat rate of 75 an hour while they try to figure it out,at your expense