Ive got a 351m in a 79 f150 and Ive been having oil presure problems for a while now. For a long time when I started the truck it would knock like 4 times then the oil pressure guage needle would go to the line that is right on the border between normal and low(its a stock guage). Then, a few weeks ago I replaced the valve cover gaskets and while I was in there cleaned out a lot of build up and other junk that was in there. When I started it up afterwards it knocked a few times, then the oil pressure went up to a perfect level. When the engine warmed up, it dropped again right to the line on the guage thats at the low end of normal. So now it does that all the time, but if I get the rpms up to close to red line then the oil pressure increases back to normal then drops with the rpms. So is my pump going out or what? sorry if this post is kind of hard to understand! Oh ya, I am using a fram oil filter if that helps you guys at all. Thanks!

 

You must have a lot of clearance in your mains. It takes a short while for your oil pump to fill up the voids left when the oil drained out of your mains while sitting.

It is normal for the oil pressure to drop when the engine warms up, due to the thinning of the oil at higher temperatures. I would be using a 20-50 oil in an engine like that to make sure it doesn't thin too much.

 

Around here ambient temps in summer are 125 plus and in winter it gets to lows of 15-18 deg F. I have successfully run many of these engines for tens if not hundreds of thousands of miles with no problems except the usual oil seeps and pressure loss with accumulated age and miles. My rules are simple.

Summer= 20w50 until they get to where they won't hold more than 40 psi hot at 3000 or more rpm. Then I go to valvoline racing oil straight 50 or 60 wt. I wholeheartedly recommend valvoline, quaker state oils, or Castrol in stock engines, with emphasis on valvoline. There are other oils out there that are good, but when you hammer on an engine beyond what it was designed for ie. cams, headers, high rpm etc. my experience is that these oils don't hold up as well. Besides I've seen several of my engines exceed 250,000 miles using valvoline exclusively and changing it at least every 5,000 miles ( all highway miles ).

Winter= 10w40 Stay off the gas till the engine stabilizes, let it warm up for a few minutes before you run it hard, otherwise if you can't do this go to 10w30 because it flows much better. But bewarned you will burn a lot more of it and your oil pressure will go down drastically when you work it hard. This for good engines.

When they wear out I run half 10w40 and half 20w50. That flows pretty good and still keeps pressure up when you get it hot.

Oils that I won't use include, Texaco havoline, Chevron, any no-name junk unless it's going into industrial engines that run at steady state rpm all the time. Even then, these oils tend to leak out of every seal and burn at two to three times the rate of valvoline or Castrol.

Oils that are expensive but worth it include Sythetics and blends. I have a Suzuki Samurai that sucks oil like mad and leaks everywhere unless it gets Valvoline Dura-Blend. Then it won't burn a drop and the leaks go to seeps. Besides if you are in extreme heat like the desert southwest these oils won't coke or degrade like regular oils do.

Additives I don't run PTFE in anything I want to keep. There are numerous threads all over the internet about this stuff. If you run it and you lose an engine, you are not the first and you won't be the last. Morey's is pretty gooey, it really helps worn out engines hold pressure and stop burning for a little while. Any of the really thick sticky gooey additives will help old engines.

Good luck.

 

yep, I replaced the pump, thinking the same thing...this HAS to do it...right???....WRONG!!!

ZIPZOOOM.

 

i dont really like to use 20w-50 oil in an engine unless its really really hot or engine is worn out. if you notice... car manufaturers nowaday specify 5w-30 for normal conditions. the reason for this is most of the wear in an engine comes from cold startups. the 5w-30 oil flows much quicker to the mains and all important parts. 20w-50 takes much longer.

 

My 2002 Mountaineer requires 5w-20 in the 4.6l, but that doesn't mean that when it has 200,000 miles and oil is leaking from everywhere and you can see daylight through the bearings that I will void the warranty by using 10w-40
Another reason that car manufacturers specify lighter weight oils is to make the gas mileage look better.
I always heard that you should use the lightest weight possible, just because of the dry start problem.

If you motor is using more than one quart per oil change, it may be time to start thinking of a higher viscosity oil.

 

Another reason for the lighter weight oil in the newer engines...
The oil control rings have a lighter tension for reduced friction. They will leave more of an oil film on the cylinder walls with heavier oil which then gets burnt by the combustion flame.

 

Thanks guys! Youve saved me some money, I was ready to go out and put a new pump in and replace anything else that I thought would help. Right now Im running 10w40, I was running 10w30 but decided to switch a couple months ago. I guess Ill step it up to 20w50 and see how that helps. Im in southern California so the weather is always warm all year round, so I dont really have to worry about changing back during the winter (although I probably will). Thanks everyone!

 

Yeah I know a guy who puts only 25-50 pennzoil in all his toyota race cars. He swears by it and those engines take a beating.