My '95 SSEi with 167K miles has a 10-15 PSI drop in oil pressure after about 5-6K miles. I use Mobil 1 10-30 synthetic and usually change it at 10K miles. Any ideas on what could be causing this? It's not likely to be sludge be the car has had regular oil changes since new, and I've run Mobil 1 in it since I bought it with 55K miles on it. There is no coolant in the oil either.

Is it a constant drop all across the board at all temperatures and RPM from what it was before or only at a certain time like when at cruising speed?

Using that oil for that long I would say no sludge as well, I run 10,000 mile oil changes on my '03 SSEi using Mobil 1 full synthetic as well and have since I bought it with 125,000 on it and now it has just under 260,000 on it and the engine is super clean.

Possibilities could range from a failing oil pressure sending unit, oil breaking down, gauge issue, or even possible bearing clearance issues.

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Ryan

2003 Bonneville SSEi - The Black Mirror SOLD!
2002 Bonneville SE - The Mutt Complete 2004 SLE interior, drivetrain, and body harness swap, ECC swap, HUD swap, black GXP wheels, GXP headlights and tinted tails - SOLD
2003 BMW 540i M Sport, 2001 BMW X5 4.4i, 2010 GMC Acadia, 2017 Grand Design Imagine 3150BH
1982 Cutlass Supreme - The fun one

The drop is consistent under all conditions; in other words it is always a little lower at idle, or when the outside temp is hot, whether the oil is fresh or has been in more than 5K miles. I don't think it's the sending unit, since that would not explain why the pressure rises with an oil change. Coolant diluting the oil would make sense, but I see no indication of that.

Verify the gauge reading with a mechanical gauge first. If it's indeed low pressure then either change it earlier so it doesn't drop or try a 5w-40 synthetic.

_________________
Boost addict


The Fleet:
'93 SSEi - Twincharged + manual Build thread
'97 Camaro - Top swap
'05 STS - V8, AWD, her DD
'92 Trofeo - Fair weather DD
'99 Montana - top swap 3800
'04 Sierra 2500HD - LLY Duramax

Current project:
Something cool, trust me.

Upcoming projects:
'92 Bonneville SSE
'87 LeSabre T-type
'67 LeSabre

Gone to greener pastures:
'84 Sierra Classic - Twin turbo 3800
'97 LeSabre - Top swap

RIP:
'86 LeSabre - pictures
'93 SE - L67

Since the gage reading rises after an oil change, I'm thinking the it's working OK, and recording the thicker viscosity of the new oil.

If that's the case, you could try installing a new filter every 5k miles and adding a quart to the system to refill it for the 10k OCI.

You might also consider adding an oil cooler. There is a spin-on cooler that ties into the coolant system you could use. With the '95 though I'd consider relocating the filter to make some extra ground clearance for that.

_________________
Boost addict


The Fleet:
'93 SSEi - Twincharged + manual Build thread
'97 Camaro - Top swap
'05 STS - V8, AWD, her DD
'92 Trofeo - Fair weather DD
'99 Montana - top swap 3800
'04 Sierra 2500HD - LLY Duramax

Current project:
Something cool, trust me.

Upcoming projects:
'92 Bonneville SSE
'87 LeSabre T-type
'67 LeSabre

Gone to greener pastures:
'84 Sierra Classic - Twin turbo 3800
'97 LeSabre - Top swap

RIP:
'86 LeSabre - pictures
'93 SE - L67

On my 10 Camaro the 10k oil changes on the monitor were changed to 5k.

This was due to the oil breaking down and getting gas soaked beyond 5k.

Lots of trouble with timing chains if you went the extended oil change with the 3.6l.

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90 SSE Bonneville Sold but not forgotten.