which oil?

Messages
69
Location
collierville, TN
vehicle and engine : Jeep with 4.0L six oil choices: maxlife 20w-50 or castrol/delvac 15w-40 reasons for other then 10w-30... slight rear main seal leak and low pressure once engine is hot and at idle. I used 20w-50 last summer and had good pressure. I will be using auto-rx with the last 750 miles of current oil change. I will also be using a motorcraft FL-1A filter. Other concern besides pressure is the leak.. would a thicker oil be less likely to leak? Also cold start issue.... temp rarely hits the teens, most of the time lows in 30's and highs between 30-50 in winter and in summer its like 70's for lows and 85+ for highs in addition to high humidity... thoughts please.
 
Messages
22,187
Location
Colorado Springs
How low does the pressure go when idling and the oil at operating temp? As long as you have, I'd say a minum of 10-15 psi, and as engine revs increase, the oil pressure increaeses instantly, you're all right with still using 30-40 weight oils. I'ver seen rebuilt engines, or even manufacturer rebuilds that have seriously low oil pressure at idle. Reputable mechanics in those cases, that rebuilt those engines and build racing engines for professional racing, indicated that as long as the oil pressure comes up quickly as rpms increase, there should be no worry. You'll get better power and fuel economy as well. It does mean that bearing clearences are not as tight, but that's not always necessarily a bad thing I guess if little to no wear is occuring, and your not spinning the bearing [Burnout]
 
Messages
917
Location
Singapore
You might want to try an ester based synthetic to stop the leak. I thought all fluids are incompressible, no matter how thin or thick? How could a thick oil provide more oil pressure? [Confused]
 
Messages
6,388
Location
Washington St.
Fluids are slightly compressible. This needs to be taken into account when designing a diesel fuel system (equal length tubing to each cylinder for equal injection timing), and brake systems (DOT5 is too compressible for proper ABS operation). The compressibility of motor oil is not the reason for higher pressure. Higher viscosity oils flowing through the same gap (bearing clearances) will develop higher pressures at the same rate of flow. Ken
 
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3,845
Ken anything including but not limited to, the oil pump,bearings,journals, filters, oil itself, temperature, etc.
 
Messages
22,187
Location
Colorado Springs
No, oil pressure on the gauge is created from oil being stacked up, so to speak, at and engine bearing, usually a main bearing I would suppose. That's why, over time, as wear opens up the tolerences on an engine, oil pressure will gradually decrease, which isn't necessarily a bad thing. Many people confuse pressure for flow. If your oil pressure has gradually gone down, especially at idle, it doesn't necessarily mean that flow to the bearings has decreased as well.
 
Messages
6,388
Location
Washington St.
quote:
Originally posted by Ken4: What creates oil pressure? The oil pump?
The oil pump creates flow. Resistance to that flow creates the pressure. This resistance comes from (in approximate order) --the pressure control valve in the oil pump assembly --the filter --the movement of the oil through the oil passages --the size of the clearances in the places the oil goes to do it's lubrication...bearing clearances, etc. Ken
 
Messages
874
Location
Pacific NW
Ken, Which pumps are you talking about? On those GM/Dodge/Jeep pumps I've worked on the pump valve is simply a relief valve that dumps back to the sump above a given pressure (75+ PSID). Below that limit there's no effect on flow or pressure. Oil never passes through the valve to the engine. Perhaps a simple difference of perspective? David
 
Messages
6,388
Location
Washington St.
David, Yes, the pressure control valve just dumps excess oil back to the sump. I should not have included the pressure control valve in my list of resistances that the oil encounters--it just maintains an almost constant pressure (once the pressure is sufficient to begin activing this P.C. valve). Ken
 
Messages
3,333
Location
Bolivia
Back to the question: On my 93 Jeep GCL I never put the 10w30 recommended. As posted on another question, used 20w50 from shortly after buying it with 50,000 miles until I changed the noisy lifters, then switched to Delo 15w40. There was no difference in leakage from the rear seal. I think that engine likes to drip. Pressures ranged a little high in the 30 F mornings with the 25w50 and I worried a little about the relief valve on the filter opening to let dirt pass. Morning pressure was lower with 15w40.
 

higney85

Thread starter
Messages
69
Location
collierville, TN
OK thanx for the Replies.. I messed around with the search and will most likely use the maxlife 20w-50 oil. Last part still has me a bit in the lurch. although someone already partially answered im wondering what the real scoop is on leakage/weight relations. Would a 20w-50 be more likely to leak as compared to a thinner oil? Logic would tell me that the thicker oil wouldnt be able to seep out of the small passage as easily. Is my logic flawed? Thanx
 
Messages
8,937
Location
SC
quote:
Originally posted by higney85: OK thanx for the Replies.. I messed around with the search and will most likely use the maxlife 20w-50 oil. Last part still has me a bit in the lurch. although someone already partially answered im wondering what the real scoop is on leakage/weight relations. Would a 20w-50 be more likely to leak as compared to a thinner oil? Logic would tell me that the thicker oil wouldnt be able to seep out of the small passage as easily. Is my logic flawed? Thanx
Keep something in mind: the thicker oil is going to give you significantly higher oil pressure during cold weather starts. If you've got seal leakage (like a rear main seal), this higher oil pressure could actually exacerbate the problem.
 
Messages
6,388
Location
Washington St.
quote:
Originally posted by higney85: doesnt a 10w-40 just break down to a 10w-30?
Current 10W-40 oils are better than the 10W-40 of years back that earned the bad reputation. Or use an API-SL or SJ 15W-40. That will be dual rated for diesel and gasoline engines and be a tough oil. Ken
 
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