Hey guys, hoping somebody can tell me what I think I may already know. Recently (like a week and a half ago) picked up an 04 F150. Long story short I knew going into buying this truck that I was going to replace pretty much all fluids and filters right off the bat. About 2 days after driving the truck home I noticed a pretty serious shudder while accelerating through a turn from a stop. Did a little research here, decided it was likely the rear end.
So I picked up 3 quarts of 75w90 synthetic oil and 8oz. Of Motorcraft friction modifier. Current temp here in KS is about 85 during the day and after changing fluids problem is 75% fixed. Is it not 100% fixed because I didn't use 75w140? Parts guys said 75w90 so I went with it, yes I know dumb. Keep in mind I do no towing or mudding of any kind. Also, yes I've done the circle 8's, 3 times now at about 10 minutes a piece.
I'm assuming I need to drain it all and start fresh with more friction modifier and 75w140 full synthetic as the 75w90 just isn't "sticking" in the clutches when it's hot. Either that or the clutches need to be completely replaced?

 

I don't know if the clutches are shot, but I definitely would switch to 75w140 full synthetic and XL-3 modifier.

 

don't forget to do a couple figure 8's in the parking lot after your refill. Gotta get the clutches to spin to get them lubed up

 

Update: siphoned out over 2 qts. of 75w90 and refilled with 75w140. Friction modifier will be here Friday and I'll add it then take her out for the circle 8s. I will update again probably Saturday. Crossing fingers the clutches don't need replaced

 

I don't think mixing oil weights is a good idea.

 

I think figure 8's are a bad idea. It seems to me that it causes unnecessary wear on the clutches. Just my smelly opinion. If you drain it completely and put what Ford recommends in it with a bottle of fm and it still shudders after driving it a bit, add another bottle of fm. The last time I changed mine I used Valvoline 75W140 synthetic that claimed to have the fm in it. The clutches were grabbing a little more than they should but there was no noise.

My 2000 F150 9.75 started chattering on turns at about 80k miles. I put Motorcraft 75w140 in it with one bottle of fm and the clutches were making noise on turns so I added another bottle of fm and that worked to eliminate the noise. But there was a TSB on that model that directed ford mechanics if the truck was under warranty to replace the clutch pack and remove 0.010" of shims. Evidently Ford determined that that model was shimmed too tight.

 

Yeah, if I hadn't gotten at least 2 quarts out I was gonna just open it back up and do it the right way again. But, I did, and with 2 full quarts of 75w140 in there and less than half a quart of 75w90 I'm not too worried about the mix. Added the final bottle of FM today and ran some circle 8's. Will update over the next few days. Hopefully shudder is gone completely now.