View Full Version : Oil change - pressure change?

March 16th, 2015, 15:27
Hey guys. Decided to do my own oil changes now, which I don't know why I wouldn't have decided to do it sooner with everything else that I have tackled myself with the XJ.

Anyway, last time my oil was changed at the typical quick lube place, Pennzoil was used, pretty sure I had them put in 5W30 as winter was approaching. They used a M3600 Engine Guard II filter.

Before I changed the oil my pressure was around 40-60 accelerating and 20-40 idling IIRC.

Today I changed the oil, figured Castrol High Milage 10W30 would be fine since my '98 has 196k on the ticker. I used a Fram Tough Guard TG-16 filter. I noticed the Fram filter is larger in diameter than the old Engine Guard II filter.

Now on to my question: with the previous oil/filter used and the new oil/filter installed should one expect a 10lb.+ drop in oil pressure? I am experiencing less pressure than I had before. I wonder if that is due to the filter being larger, or to the new weight oil. I do not see any leaks anywhere.

One other thing I noticed is that the oil cap on the engine seemed to just spin around and not positively stop as if it was fully seated when I put it back on. I have had the cap off before and never remember it being like that when I put it back on. Is that normal for the cap?

Any ideas? Thanks guys.

Rick Anderson
March 16th, 2015, 18:56
You went to a thicker oil*, so if anything the oil pressures for the same conditions should be higher, NOT lower.

The earlier Penzoil wasn't synthetic was it? I change from synthetic to conventional could drop oil pressure a bit, sometimes none.

The size of the oil filter will make no difference. The oil fill cap being loose will let in dirt, but until that dirt does damage, it will NOT effect oil pressure.

That is if the conditions are all the same, oil pressure will go up/down with engine rpm. The temperature of the oil is huge and the temperature of the engine and air temperature, the hotter the lower the pressure.

So if the weather is quite a bit warmer than the last time you noted the oil pressures, that might explain it.

The Orange Fram Filters are lowest rung of quality, I would never touch them, although you're NOT guaranteed to have problems if you use them, problems occur way more often than with better quality oil filters. The pictures of the cut open filters, the other color, better Fram filters look like the quality of most of the others, but NOT nearly as good as some of the best filters that are only a few dollars more. I won't buy any Fram filters because if they put out the crap like the Orange filters, I won't trust their other stuff. If you can't find another explanation for a 10psi drop, if it was me, I'd get a wix, hastings or purolator filter and swap it out and see if the pressure comes back.

*People will say 5W30 is the same thickness as 10W30 when its warm, the numbers are approximate and it doesn't magically jump up at a single temp, it reflects approximates at either end of a curve. So 10W30 is thicker than 5W30 at about all temps, but not a whole lot.

March 17th, 2015, 06:03
Can I swap the filter out without losing all the new oil I just put in?

Rick Anderson
March 17th, 2015, 06:44
You would just loose the oil in the filter, depending on its size, that would be a 1/4 to 1/2 a quart. You should be using a funnel to fill the filter with oil before installing it, so it reduces the time the engine runs without oil on startup after an oil change anyway.

AMSOIL claims their oil will last more than a year in a motor, like 25,000 miles, but they recommend you change the filter without changing the oil halfway through. So changing a suspect filter without the oil is NO big deal.

If the oil pressure warning light comes on during idle, although its considered acceptable, its NOT a good sign. If you've got 20psi or just a little less at hot idle, you're ok, 60psi at higher engine rpm is good, if your at 40psi or less with the engine near red line, yea, a good motor should be making more than that in oil pressure. Most low oil pressure lights come on around 10psi.

March 17th, 2015, 07:09
Sometimes, if it's run too long, it can start to thicken after it shears out of grade. But we're talking like 15K+ on an oil not designed to do so.

What pressures are you seeing now? Can you confirm that they didn't knock the oil pressure sensor lose?

March 17th, 2015, 07:15
The Orange Fram Filters are lowest rung of quality,

I can't emphasize this enough. I was having pressure issues after a "jiffy Lube" type oil change. I swapped filters just to see. My oil pressure came right back to normal just changing my filter from a Fram to a NAPA gold filter (Wix).

March 17th, 2015, 07:40
I picked up a purolator classic filter and a quart of oil a lil bit ago. Been driving around observing and my pressures about 50 driving and 30 idling. Been steady so far. If I have another issue like yesterday I'll swap this filter out with the purolator.

Would the change from the Pennzoil conventional to the synthetic blend of the castrol high mileage offer a bit of a drop. From what I understand full synthetics run lower pressure, not sure bout the blends.

Rick Anderson
March 17th, 2015, 10:56
Puralator is a good quality filter, Wix is better, I like Hastings and have cut them open and they appear to be just as quality as Wix.

We can go around for 10 pages of posts about the differences in conventional and synthetic oil and blending them, how synthetic flows better even though its the same viscosity and what that means, etc, etc. Basically the answer is no, viscosity is the measure of flow, 10W30 "should" give the same flow regardless of brand or type, in reality there are differences among brands and the differences in the properties of synthetic can make a little difference, but NOT as much as your seeing. Usually people see a couple of more PSI, especially in older more worn engines, than see less when they switch to synthetic. There are arguments how it might be less, someone might have seen it, but again, we are talking a couple of PSI which a normal variance.

Someone mentioned old oil sheering down and thickening up. Although it is possible, usually oil sheers down and loses viscosity and gets thinner, I usually notice a slight increase in oil pressure after an oil change, we're talking a couple of PSI, barely noticeable. Thickening instead of thinning certainly is possible, especially cold weather, and he did say it would have to be overused by 10's ok thousands of miles, I could totally see an oil degrading to the point where it starts thickening after its thinned down from sheering and degrading.

March 17th, 2015, 17:38
Drove home this afternoon and the pressure dropped again so when I got home I spent the time that it was cooling down replacing the spark plugs.

Eventually I swapped the Fram Tough Guard for the Purolator and topped her off. Noticed that I gained about 5-8 lbs of pressure. Guess I'll stay away from the Frams.

Now to wait for my heater core and evap core to arrive to start on the real fun...

March 18th, 2015, 08:47
Cheap oil filters strike again :(

March 19th, 2015, 11:05
Strongly agree with the choice of oil & filter (Castrol & Wix).
Also I experience oil pressure changes from the wiring and connector on our 97 & 98 XJ's.
The location of the sensor's connector is such that it is easily damaged by people who do speedy oil changes & are not careful about the wire & connector. This can also cause oil pressure drops when turning corners, when accelerating or stopping, the engines shifts slightly moving the oil sender wiring / connector.
If this is an issue, moving the wire and connector slightly will restore pressure, but the only lasting fix is to cut off ~12" of wire & replace it and the connector.