I have a 4.6L V8 1997, just over 110K on the clock. The lump itself is a workhorse, I am very pleased with it. There are some design problems you may want to think about. The plugs on cylinder 2 side will foul in severe wet weather. This seems especially true if you have installed short thread plugs as these provide an extra small well for water to sit in. Of course the water should not get in the plug wells, but more often than not, the plug caps are not fitted properly, or are just plain worn at the edges which allows rain/mud etc to get around the plus and plug caps. Most garages will tell you you need new wires, but really all you need to do is to pull the plugs, clean the wells, clean the plugs and replace. I also put some high melting point grease around the base of the caps to prevent water entry. The other design problem I noticed is the plugs at the back of the engine (cylinders 6 and especially 8) are very hard to get to.

 

I have a '99 F150 4WD 4.6L 5speed manual tranny. It has 120,000+ miles and no mechanical problems with the engine.
I would be more concerned about the Mazda manual transmission/clutch than about the engine. This seems to be the weak link. I have not replaced my clutch or tranny but it has always been hard to shift and difficult to get into 1st gear.

I am lucky to get 15 mpg.

 

I have a 4.6 with the 5 speed and have never had a problem with either one, i will admit the tranny could be a whole lot better but i have no problems shifting the Loooooong gate. 18+ miles to the gallon.

 

I have a 97 Supercab with 146K. I always get 20-22 MPG on a trip, and can get 23-24 MPG with a tailwind. I have replaced sparkplugs and all O2 sensors, and beside that just oil and various filters and a few light bulbs, wiper blades, a PCV valve just because I wanted to change it, and fixed 7 flats. The headlight was taken out by a flying rock at 140K; not the truck's fault. A coyote ripped off my license plate holder at 70 MPH (you should have seen the coyote crap on the bumper). The motor is the last thing I worry about. I think you will have more tranny problems, but I haven't had any of those either except for a minor fluid leak due to some loose bolts. I still have the original serpentine belt on mine, and it looks really good......couldn't find any cracks at all and well within wear limits. My old Mercedes-Benz 300D was a maintenance hog compared to my F150, but I'll wait until my F150 gets 300K on it to make a final judgement about that.

 

I've had three 4.6Ls. the latest one is in a Mustang GT 2003.
Bunch of the mustang guys w/ 4.6Ls abuse the hell out of their engines...drag racing, motor cross, superchargers, nitros ox., etc. They've even gotten 1500 HP on a stock block (forged internals). Strong engine to say the least.

 

What is your take on synthetic oil? Should I be using it?

 

You will notice a search function at the top of the page. You can type in keywords like "synthetic oil" and read all the pro's and con's to your hearts delight.

My engine leaked/used oil when I tried the Mobil One so I switched to Amsoil and the leaks/oil usage stopped. I did not go back to Mobil One so I can't say that it was the cause of the oil leakage/usage. I am sticking with the Amsoil for now.