Oil pressure drop after using GM Top Engine Cleaner

I have a '96 XJ with about 66k on it. I did a tune up yesterday, but before I did that, we poured a can of GM Top Engine Cleaner down the throttle body, per instructions on the can. After sitting 2.5 hours, the XJ fired right up without problem, but was running a little rough. I thought it was supposed to be hard to start?

Well it smoked for 4 or 5 minutes, and then I changed the oil on it. I was previously running Amsoil series 2000 0w30, but thinking this might be a little too thin, I switched to Amsoil series 7500 10w30 (supposedly thicker?). I'm going to switch again to 5w40, but I didn't have any on hand.

I have now noticed my oil pressure is about 10 pounds lower, at idle, than it was with the 0w30, and before running the cleaner. It is also about 5 pounds lower at speed, and at cold idle.

Right now I'm getting about 45ish driving, and about 25ish idling. Could it be that the engine cleaner cleaned up enough gunk that the oil now has a larger space to travel, hence the lower pressure? The engine was pretty clean to begin with. Or do you think it was caused by the 0w30 to the 10w30 switch? The Amsoil rep said 0w30 is safe to use in place of all 5w30 and 10w30 applications.

What do you think?

First off, the oil pressures you are getting are well within tolerances, so don't worry too much. Anything over 13lbs at idle and 35 at cruise is within factory tolerance.

One other thought: did you use a different brand of filter? I've noticed a good 5+ pound difference just from one filter to another.

I have been using only Amsoil filters. I know I am well within factory specs, I would just like to know what has caused the drop in pressure, especially after switching to a thicker oil.

Within specs or not, I'm not really comfortable with having only 25 PSI at idle.

Some carbon may have been dislodged from the rings and gone into your oil filter or, more likely, into your oil pump pick-up screen. Try changing the oil filter first.

Amsoil S2K 0w-30, kinematic viscosity @ 100°C = 11.2 cSt
Amsoil XL-7500 10w-30 kinematic viscosity @ 100°C = 10.2 cSt

The 30 weight range runs from 9.30-12.49. You can see that the XL-7500 10w-30 is on the thin side, which probably explains your lowered pressure. Also keep in mind that Amsoil has a reputation of thickening as it's used, so the oil you drained may have been even thicker than 11.2 cSt. Those two things combined may explain the pressure difference.

I just finished a 10k mile run on the Amsoil 5w-40 AFL. My pressures were about 3-5 PSI higher at the end of the run vs. the beginning.

Sparkman said:

Amsoil S2K 0w-30, kinematic viscosity @ 100°C = 11.2 cSt
Amsoil XL-7500 10w-30 kinematic viscosity @ 100°C = 10.2 cSt

The 30 weight range runs from 9.30-12.49. You can see that the XL-7500 10w-30 is on the thin side, which probably explains your lowered pressure. Also keep in mind that Amsoil has a reputation of thickening as it's used, so the oil you drained may have been even thicker than 11.2 cSt. Those two things combined may explain the pressure difference.

I just finished a 10k mile run on the Amsoil 5w-40 AFL. My pressures were about 3-5 PSI higher at the end of the run vs. the beginning.

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Should I expect the 5w40 to be thicker than the 10w30? Or should I just switch back to 0w30?

My understanding is that the 4.0 likes a 40 weight. I would use the 5w-40 unless you see some really cold starts. You don't say where you are. I am using the 0w-30 because I expect to see at least -15°F this winter. The 5w-40 will be going back in next spring.

Temps around 0 degrees are not uncommon around here in the winter. 5* to 10* mornings happen a lot more frequently however.

And I assume you are using the European Engine Forumla 5w40? Do they make another type of 5w40, or is this just a fancy name?

Yes, the Euro 5w-40. Nothing special about it AFAIK, just that 5w-40 tends to be spec'd by european manufacturers. I had great oil pressure, quick pressure up, and quiet running with this oil.

Most of my really bad cold starts are in the 5°F to -5°F range, and past years I have always used 5w oils in the winter. I probably only see a few -15°F cold starts a year. Never had a problem. Either 0w or 5w should work fine for you. I would consider 10w marginal at 0°F.