I gotta a '94 F150, 5.0/auto with about 170,000 miles. I'm thinking about switching to synthetic oil. I heard somewhere that using syn. on an engine with that many miles, it might start to leak oil. The truck don't leak a drop right now, so I don't wanna cause a problem.
What brand do ya like best? I've used Mobil1 on a different truck, but it was much newer.

 

With that many miles, I wouldn't switch to synthetic. I wouldn't even recommend changing oil brands. The only time to do that is when the vehicle is new or if you buy a used car with lower miles definitely less than 100K probably more like 50K or less. The main thing is just make sure your oil stays clean. You can however use the high mileage version of what ever brand you are using. I started doing that before it rolled over 100K and now it has 220K Mine is a 95 302 with no smoke and only the rear main seal leaks.

 

I'm a big believer in synthetic, I used Mobil 1 in a couple of cars we bought new. After tearing one of the engines partway down to do timing chains (which I found out it absolutely didn't need) at well over 100k miles it looked like it had about 10k miles on it, clean and perfect looking as can be, factory specs on bearing clearances, etc..

With that said, I never plan on using it in my F-150 because I bought it at 164,000 miles. Regular oil has a mixture of different sizes/lengths of molecules. As seals get older, the longer, waxy molecules form together and kind of "plug up" any places where there would be small leaks. Synthetic is all the same sizes/lengths of molecules, which are smaller. The waxy buildup is cleaned away and leaks that were there just from age start leaking.

Point is, I really like syn but even I wouldn't use it in an old truck. Just keep the oil changed with a good quality conventional.

This site is good to get into the "fine points" of lubricants: http://www.bobistheoilguy.com/

 

my last van, 1976 e150, bought it in 02, 100,000 miles
used it for 4 years and 50,000 miles. the only oil in it
for me was 10w40 mobil 1
current van i've had 6 months, 1996 e250, 201,000 miles
when i got it. 10w40 mobil 1 again and is now down 1 quart with 6,000 miles
if it didnot leak before it should not leak after starting
with synthetic

 

Right now I'm using reg Mobil , the stuff they say can go 5,000/7,500/15,000 without a change, cause thats the baseline stuff. I can't find just regular Mobil no more. I don't trust that at stuff though. I buy the 5,000 mile stuff and change it at about 3,500-4,000 miles. I've tried using the 7,500 mile stuff in my car, but even after 5,000 mile it starts to sound a feel alot more rough. I don't think I want to chance it with syn. It runs fine and I get a happy feeling when I tell people how many miles it's got and no leaks. What do yas think about that high mileage stuff? Does it improve anything? Is it worth the price.

 

I've been running syn in my 89 F-150 for over 15 years. I ran Mobil 1 for about 10 years with 6000 miles or 6 months btw. changes and 3000 miles between filters. (A good filter is very important) Now I run Walmart Brand Syn. b/c the truck is older and the oil is cheaper. No complaints so far with 4k-5k btw. changes w/o changing the filter. With over 135k it doesn't burn any oil and I havn't had a single engine problem.

SYn blends will often provide the same protection but I feel that it doesn't keep you engine as clean as pure syn will. Another popular but expensive brand of syn is Amsoil. I run it in my motorcycles.

However, after so many miles, I wouldn't suggest changing to syn or even switching brands. As gatesj mentioned, the damage has been done and switching to syn will most likely clean out any build-up and cause your engine to start burning oil. The best time to switch is when the vehicle is under 50k but after the engine break in period.

 

I've ran synthetic in my F150 5.0 from new to 160,000 mi pull the motor fix rear seal and other miner things took the heads off and the factory cross hatch were still verry visible. so good stuff