Low mpg after oil change

Messages
53
Location
Santa Cruz, CA
I changed the oil for the first time on this car (a 1990 300zx non-turbo)last weekend, swapping out from what the previous owner told me was m1 5w30 for Castrol GTX 10w30. The old oil had 5k of short trips on it, but wasn't too nasty. Over 4 quarts came out between the filter and drain. I put in 4.3 quarts of the GTX, which the cars manual asks for 4 and 3/8 quarts. A week later, I notice that the MPG has dropped at least 3 mpg, with the guage showing more than half empty after only 115 miles or so. Before the OC I was getting close to 280 miles out of 3/4 of a tank. What gives? There may be some sludge in there which is causing this loss, though I don't believe the oil level is too high( dipstick has always been unreliable). Oil pressure is higher than before, over 60 psi idle at cold startup. Any suggestions or comments would be apprieciated, thanks.
 
Messages
15,874
Location
NE,Ohio
1/2 a tank isnt long enough. come back after 1000miles of worse mpg. Offhand maybe the m1 was on the thin side and the castrol is on the thick side of the grade.. but it shouldnt be 3mpg.. maybe 1mpg or so at most.
 
Messages
23,894
Location
CA
It's rare that I hear such a drastic difference, but the change in mileage could easily be attributed to the higher start viscosity of the new oil. The old oil could've sheared down significantly with use, and the new oil is now "thicker" (as it was in the beginning), hence the mileage difference.
 
Messages
23,591
My gas mileage has been horrible lately. I gas up with top tier gas only, my O2s are pretty new, the MAF is fine and I don't drive with my lead boots on -- it must be the gas quality. I know my engine reacts negatively alcohol, so I suspect high ethanol or methanol content is to blame. Santa Cruz, eh? I just got some of Rotten Robbie's finest a couple weeks ago.
 
Messages
6,171
Location
Santa Barbara, CA
Quote:
Try Amsoil 0W-30, MY MPG PICKED UP 5 MPG USING THIS OIL.
Quote:
use AJ AMATUZIO oil and watch your mpg exceed anything you had before. you get what you pay for.
Magic Oil
 
Messages
9,427
Location
Pensacola & Vero Beach FL
I concur with the winter gas theory. Somehow, it's a crime for one of us to drive under the influence of alcohol -- I think it's a crime (or at least a cryin' shame) for automobile fuel to propel a car under the influence of alcohol!!!
 
Messages
30
Location
Long Island, NY
Quote:
Try Amsoil 0W-30, MY MPG PICKED UP 5 MPG USING THIS OIL.
Yeah but you just paid your gas savings up front by buying an oil that costs 3 times as much.
 
Messages
1,779
Location
Central Iowa
I doubt very much if you're seeing that much variation just from oil. As an owner of a 1992 300ZX I can assure you that there are too many variables in driving habits to try to trend fuel mileage consumption over 1/2 tank of gas. Just getting after it a couple extra times can account for that much of a fuel consumption increase.
 
Messages
1,383
Location
Agrestic, CA
Quote:
I concur with the winter gas theory. Somehow, it's a crime for one of us to drive under the influence of alcohol -- I think it's a crime (or at least a cryin' shame) for automobile fuel to propel a car under the influence of alcohol!!!
In California, the cars out here are all alcoholics. It's time to take the CARB to an AA meeting.
 
Messages
2,698
Location
Silicon Valley
You shouldn't try to figure MPGs based on the fuel gauge, especially at in-between points like 1/2 or 3/4 as the guage is probably not linear or repeatable. Use the number of gallons on the pump read-out at fill-up. Or is the trip computer giving you the MPG figures?
 
Messages
10,906
Location
Nokesville, VA
Even the number of gallons on the pump at fill up can vary. I've had pumps shut off prematurely and I've had pumps that didn't shut off even when the tank is full.
 

beercollector

Thread starter
Messages
53
Location
Santa Cruz, CA
Thanks for the input- the gas could definetely be a factor, I filled up at the 76 by Dominican the day before the OC. I always go there late evening/night ish and always pump from the pump furthest to the east on the right side of the island closest to Soquel ave. It's just that beeing able to see the gas gauge needle start to drop on short drives is getting me started on walking to work during the day and then taking one drive at night to get everything else done. Gas prices are now under $3 here (gasp) it's not the money thing, it's thinking that my car isn't doing right that bugs me.
 
Messages
1,383
Location
Agrestic, CA
Quote:
I always go there late evening/night ish and always pump from the pump furthest to the east on the right side of the island closest to Soquel ave.
Okay, I'll bite. What does the pump being further east have to do with anything? Are you assuming it is cooler and therefore more dense? (not trying to be rude - I apologize if it sounds that way. I am actually curious)
 
Messages
39,805
Location
Pottstown, PA
Quote:
I think he is saying that he fills up at the same pump at approximately the same times of the day.
That was my take on it I used to do this too. My wife commuted past very expensive gas. I commuted by substantially cheaper gas. We worked somewhat different schedules. So I drove her jeep to the same gas station one day a week ..the same pump ..same pitch/slope..every time. That's how I tracked the mileage for every fill up. I did that for over two years. Although that was a rather detailed description
 
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