Just got a quick question about oil presure my 1977 F250 4x4 with the 460 engine.

I've heard about the 10 psi per 1,000 rmp rule, and although my truck never drops below this I'm still not sure my oil pressures i adequate. I've never owned an old vehicle before my truck so I don't know what is bad and what isn't.

(all of the oil pressures I mention are when the engine is hot, after driving on the highway for about 8 miles)

Oil pressure is about 12-15 psi at idle when in drive (idles about 650 rpm). When in neutral, it idles at about 750 rpm and oil pressure is about 16-20 psi. Is this an acceptable oil pressure?

When I drive on the highway, pressure varies between about 30 and 45 psi. This is when I'm driving at about 55 mph and the engine going at about 2,250 rpm. When I hit the gas (like I'm going up a hill or passing another car), the oil pressure tends to be closer to 30 psi, and when I back off the gas or am going on level ground it is about 40 psi. Is this normal for oil pressure to drop slightly with engine demand? Is this oil pressure acceptable?

The oil is 5w/30. Should I use a different oil weight?

Any help would be greatly appreciated. This is my first old vehicle, so I'm not used to dealing with issues like these and I'm a bit paranoid. Plus, I've had some really lousy luck with this truck (transmission, carburetor, and radiator had to be replaced in the last 6 months) so even minor issues concern me a bit. I'm in a bit over my head with this truck, but I like it a lot and want to keep it running without spending too much money on it.

 

Those numbers are fine.

The rule you cite is accurate.

 

As already stated 10psi per 1000rpm is good. Sounds like you have plenty of pressure. There's plenty running around fine with lower.

 

Your good to go.
I know how you feel though.
Wait till your out on a trip at a steady 65 or 70 and see your gauge on the down low.
Pucker time.

 

If you would like to see more oil pressure, use a thicker oil. Don't know what you are running, but I run 10w-40 in all my vehicles in Massachusetts year round.

 

Sounds to me like you have worn main bearings (and likely rod bearings as well). Under power, the crank is pushed against the lower main shells and away from the uppers where oil feed holes are, so oil pressure drops ...
... but under coast with throttle closed vacuum is high and the crank is pulled up against upper main shells and restricts oil feed holes and oil pressure rises.
You are using the wrong weight oil, that oil is for late model engines that are built to tighter tolerances and it's use is in part, related to getting higher mileage scores for auto makers. At minimum, you should be running 10W30 or 10W40 ...
.... and IMHO, a 15W40 (it's a HD truck oil) or 20W50 wouldn't hurt it at all (and if I suspected worn bearings like I do here, I'ld run one of these later two).

 

5W/30 is a Honda or some modern engine with tight clearances, I think the 15W40 is the best, it's a diesel oil (Delo 400 is my favorite) you have 3" main bearings give it something thicker, unless you are below zero temps, then 10W40 maybe.

 

Agreed, I wouldn't put 5W30 in something like this (missed that the first time). I run 10W30 which is the lightest I would go. I ran 20W50 in my Galaxie to keep the pressure up because it was approaching zero PSI.

 

Thanks for the responses. I just realized I am definitely running the wrong oil. I had the shop change my oil when my transmission was rebuilt 1 month ago (I was feeling a bit lazy) and they put in 5w-20. I have to imagine that isn't helping.

I'm going to change the oil next weekend. I think I'll try 10w-30 for now, but maybe 10w-40- I've got a week to think about it.