Our 93 Sub has 340k on the original 5.7, only need to add a quart about every 2000 miles and runs fine. As expected, as it warms up, oil pressure drops. Starts off cold at 45psi or higher at idle. Once warmed up, holds 15psi at 60 mph, drops to 5psi or lower at idle -sometimes even triggers the check gauges light. Never any noises associated with the low pressure. So my questions are:
1. Original spec calls for 5-30 oil, should I change to 10-40 or 20-50?
2. Will increased viscosity increase pressure; but could this increase be harmful at cold motor start up?
3. And finally (here's ignorance on my part) I understand that the pressure drop is due to "wear and tear & increased tolerances" - but never have been given a satisfactory explanation as to why the pressure reads low if the oil pump is still pumping, and more importantly, if the oil is still flowing and lubing and no noises, does the low pressure really matter?
Thanks for your input, Red

 

1 quart in 2000 miles is not much considering.


for pressure to exist, you need restriction. As you have stated the tolerances are greater, so it only follows that the pressure drops.


a rule of thumb for oil pressure minimum specs is 10psi for every 1000rpm. if your above that, your ok.
you could test out the 10w oil for your oil pressure issue. I agree; concern for the 20w oil would be during cold winter start ups.


the only way to know where the issue lies is to check the pressure at the pump and at the sending unit.


Sounds like that truck has been fantastic.

 

So sounds like 5 psi at hot idle and 15-20 on the freeway at 60-70 mph ain't too bad for my old friend. Thanks for the input. I will try 10-40 when I change it tomorrow.

 

Thought I would update this since changing to 10-40. After a 30 minute freeway drive, pressure at 15-20 at 60-70 mph and more like 5-10 at idle vs former 5 or less. Also seems to jump to a higher pressure now when you get on the throttle more. So a little improvement and not triggering the check gauges light anymore. So should be ok for another 2-300k or so I guess as long as I have heavier wt oils to fall back on!!

 

I'd have no problem recommending the 20w-50w year round, if you just wanted to see more on the gauge.
Even in cold weather, with your "earned" clearances, there won't be a problem with a slow crank.

As stated, fantastic truck !

 

I had a crazy concoction at 170k on my 2.8L v6..

1 bottle Kendal straight 70w
2 bottles 20w50 castrol
2 bottles 10w40 or something light.

At least the bearings were quieter.

 

I'll bet you tend a really mean bar!!

 

Update - recently changed the 10-40 to 20-50. Now hot idles at 10-15 psi and freeway @ 2000rpm 25-30 psi. Cold start 50psi. Just like a new truck again! Odometer at 343.5k.

 

good to know