My '95 Bonneville 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'* 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.

10K miles? Wow man, you are pushing the limits even for synthetic.

The 10-15 psi drop is normal for just about any engine with OLD oil. Its because the oil is losing its viscosity and can't hold pressure or the oil is getting so thin, its leaking past bearings.

And why are you running 10W-30? For the L67, mfgr recommends 5W-30.

Go change your oil, you will immediately notice a change in oil pressure. Then, change your intervals to 5K miles.

And when you say its not likely the engine has sludge buildup, 10K oil changes IS not regular oil changes. Sir, do not believe the synthetic hype.

Thanks Mike. The owner'* manual recommends 10-30, I just double checked it. 90% of my driving is on the Interstate, so I didn't think 10K was too long an interval. I don't want to get into an oil argument, but people run Mobil 1 as high as 15K in these engines and get good results from the oil labs about viscosity, and other things they anallyze. But, your theory makes more sense than anything else, I just don't understand why the oil would be breaking down that fast.

It was on the Series III 3800 that they started recommending 5-30.

Funny, my 97 L67 oil cap specifies 5w30. Which is what I based my post about the recommended weight.

yeah, mine'* a Series I and it says 10-30 on the cap and in the manual. Lower viscosity on the lower end wouldn't effect my issue, I don't think. Whatever is going on, I will start changing the oil at whatever mileage I see the pressure starting to drop. Only an oil analysis will tell for sure.

i have always been on the side of frequent changes keeps the motor clean, the oil has cleaning packages added that need to be changed too. i have run regular Valvoline, changed every 5k in every car and keep my drivers 2 to 300k and never had a wear failure. i know a few that use mobile one that cant say that. i sometimes have to change it once a month but its the cost of doing buisiness if you want to make the car last that long. you can also get it for 16-18 bucks an oil change jug at walmart

Whatever'* going on, I plan to change my oil as soon as I notice the oil pressure drop. It doesn't make sense because there are plenty of people who use Mobil 1 and change it at 20k, and their engines so little sign of wear even above 200K miles. Hundreds of oil analyses verify Mobil 1'* ability to stand up beyond 10K miles. That why my situation has me baffled.

Your Mobile 1 may be doing its job preventing premature wear, but we don't know if its viscosity is breaking down sooner.

And Larry, your not the only one on here that has "seen" weird oil pressures. Your like the 20th one I've seen with the same issue. Change the oil, and OP is right back to where it suppose to be. 3-5 months down the road and everyone notices the pressure starts to drop back down again.(varies with mileage)

So, here'* a game plan. Change the oil with your normal brand, and change it out again at 5K for the next 2 changes. I betcha it will change.

Thanks Mike, that'* my plan.