Oil pressure a bit low after running

[newbomb]

Hello,

I have an 84 A/S -- 860 hrs.. Engine is still stock outside of minor maintenance such as raw water pump and distributor. I have noticed this summer that the oil pressure seems low at idle after running for awhile. It drops to about 10-15 psi at around 800 RPM in neutral. Putting it in gear seems to cause it to drop a tiny bit more (to around 10 PSI).

It runs great, and the oil pressure comes right back up to normal when I bump the RPMs up over about 1000-1200. Still runs crazy fast and good at WOT.

Is the old 351 just getting a bit tired? Should I run a different oil now -- I've been running 5W30 because that's what I use for my other engines. Any other thoughts on what I should do -- or should I not worry about it?

Thanks,
Todd

[backfoot100]

Most older ski boats prefer a heavier oil. 10-40 or even 15-50. You also should maje dure that it has enough ZDDP to jeep flat taopets from rounding off. Modern 10-30 oils don't have the ZDDP content.
Most O job use VR1 10-40 or Mobil1 15-50.
The oil pressure you're seeing isn't terrible. It's still more than sufficient. General rule of thumb is 10PSI/1000 RPM.
A heavier oil will bump it up.

[RonT]

As usual I agree with motor head Backfoot, I use 10-40 here, and add a bottle of STP oil treatment (heavy in zink)

[PaulM]

An oil as light as 5W-30 was probably never recommended by the Manufacturer of your engine.  Change the oil to a Mobil 1 15W-50 or Valvoline 20W-50 (or equivalent product) and recheck your idle oil pressure. 

If you still have 10-15psi oil pressure at idle you may have premature engine bearing wear (i.e. someone infrequently changed the engine oil). 

If, after changing to a heavier weight oil, you continue to have low oil pressure, all is not lost.  As Backfoot opined, only 10psi/1000 engine RPM's is necessary.  Keep the oil fresh (i.e. changed) going forward and your engine may last many years.