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Old 06-25-2010, 08:23 AM   #1 (permalink)
Duramax Dad
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5/40 oil when towing

Do any of you guys use a 5/40 synthetic oil when towing heavy loads in the summer time? I tow around 10,000lbs and would like to switch to synthetic oil, but the owners manual states, not to use 5/40 oil in temps above 100. The temps around here have been in excess of 100 the last couple of days.
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Old 06-25-2010, 09:16 AM   #2 (permalink)
IGO1320
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Quote:
Originally Posted by Duramax Dad View Post
Do any of you guys use a 5/40 synthetic oil when towing heavy loads in the summer time? I tow around 10,000lbs and would like to switch to synthetic oil, but the owners manual states, not to use 5/40 oil in temps above 100. The temps around here have been in excess of 100 the last couple of days.
No worries. First number is "cold weight" of the oil, second number is the weight of the oil the engine sees during normal operation. There is no engineering downside to running 5W40 synthetic. I use 5W30 and I have hauled some "heavy" loads. Got 106K on mine now no problems. Last truck (gas) I ran 0W30 and it had 243K on it with no problems, same oil pressure then as it was new. You will have no problems with 5W40.
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Last edited by IGO1320; 06-25-2010 at 09:22 AM. Reason: mistyped info.
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Old 06-25-2010, 09:22 AM   #3 (permalink)
srjacob
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Synthetic is great in cold & hot weather. That is one of the advantages because the attitive package does not break down under extreme conditions.
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Old 06-25-2010, 11:20 AM   #4 (permalink)
Heath
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Two separate issues...use 5/40 and/or use synthetic. As mentioned the 5/40 should work just as well as a 15/40. As for synthetic, I'll be the first to tell you sure you'll see an advantage, but depends on what you are looking for. This has been hit hard in other threads so I won't go into details..save time on the rebuttals
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Old 06-25-2010, 11:44 AM   #5 (permalink)
Vin63
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Quote:
Originally Posted by Duramax Dad View Post
Do any of you guys use a 5/40 synthetic oil when towing heavy loads in the summer time? I tow around 10,000lbs and would like to switch to synthetic oil, but the owners manual states, not to use 5/40 oil in temps above 100. The temps around here have been in excess of 100 the last couple of days.
I tow quite heavy especially in the Summer here in SoCal, as well as throughout the Southwest, and use 5W40 synthetic with excellent UOA reports.
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Old 06-25-2010, 11:53 AM   #6 (permalink)
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Attached is my latest UOA on Rotella T 5W40 synthetic. I towed a 12,000 5er from Texas to Fairbanks AK throught the Alcan Highway and I believe that the wear numbers are pretty good.

Kevin

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Old 06-26-2010, 07:00 AM   #7 (permalink)
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When I had my 02 LB7 used 5-40 synthetic on two oil changes and did notice a decrease in idle oil pressure.Maybe 4 psi which returned when switching back to 15-40.
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Old 06-26-2010, 11:55 AM   #8 (permalink)
IGO1320
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Originally Posted by SAYWHAT View Post
When I had my 02 LB7 used 5-40 synthetic on two oil changes and did notice a decrease in idle oil pressure.Maybe 4 psi which returned when switching back to 15-40.
With oil at operating temp there should not be a difference in oil pressure. Cold start up there will be, the 15W40 should be higher until it warms up.
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KYB shocks (stock sucked)
Autometer Boost/Pyro gauges
EGR blocker and (Kennedy) finger stick
Transgo Jr. installed at 77325 miles
Kennedy Pump Rub plate installed, 98060 miles
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Old 06-26-2010, 12:47 PM   #9 (permalink)
DURAtotheMAX
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Ive found 5w40 runs too low oil pressure in my truck.
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Old 06-26-2010, 01:57 PM   #10 (permalink)
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Quote:
Originally Posted by Duramax Dad View Post
Do any of you guys use a 5/40 synthetic oil when towing heavy loads in the summer time? I tow around 10,000lbs and would like to switch to synthetic oil, but the owners manual states, not to use 5/40 oil in temps above 100. The temps around here have been in excess of 100 the last couple of days.
If you live in a climate that gets that hot you might as well stick to a 15W-40 no real need for a 5W cold weight especially when towing heavy.

The cold pour point of Amsoil's 15W-40 CI-4 oil is -44F and the CJ-4 15W-40 is -40F I think that would be ok in your climate (not sure where you are) since your getting 100+ temp days I bet your winters aren't -44F for any length of time. Go with a 15W-40 and be done with it.
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