Hello all! First I'm sorry if this has been beat to death but I didn't find much in my searching if you have a link feel free to post it.

My question is should I change my oil weight in my early 99 7.3 from 15w40 to 5w40?

The truck has about 300k on it. Typically go about 8k miles on an oil change. Pretty heavy use, traffic, hauling, towing, dirt roads. I try to run full synthetic but lately it's impossible to find. And 30$ a gallon when I can find it.

It has always been a beast on cold starts even with new glow plugs. I usually plug in the block heater below 35 degrees otherwise it's a loud angry sounding engine.

I have an 04 6.0l as well that I run 5w40 syn in so it would be nice to have the same oil for both.
I just don't know if it would be good for the 7.3 systems. Or if dino 15w40 would be best to stay with.
Thanks guys!

 

No worries at all if you want to do it. It sounds like you might be looking at a need for new injectors in the near future if your are having cold start difficulties and the GP system is all functioning properly. A move to 5w 40 should help a little with it in the mean time.

 

Switch to 5/40 t6 rotella.

Cold starts are a mystery but that solved my northern Michigan romping and I'm not looking back!!

Denny

 

worn injectors retard timing making it hard to start


2 or more dead Glow Plugs can make it romp a lot when cold too


Slow starter can wreak havoc as well as dirty battery connections making you think your starter is bad
https://www.ford-trucks.com/forums/1...ou-cranky.html

 

Thanks for the replies!
the injectors have about 120k miles on this set so I could see them being weak. I do have a bully dog unit on the truck for mileage.
The glow plug system is all brand new aprox 1k miles. Batteries have tested good recently(die-hard platinum) parts are clean and the starter was replaced last year but I realize that could be an issue.
i guess if the injectors are ok running 5w40 My only concern would be loose tolerances on this higher engine that 5w40 might not work as well as heavy 15w40.

 

It sounds like some tests are in order for the injectors. It did just get a new hpop recently( un serviceable port was leaking)

 

5W-40 and 15W-40 are the same once the engine oil is at operating temp. They are both 40 weight oils at 212°F. The only difference is the cold temp viscosity, obviously the 5W is thinner and will flow better until the engine warms up. The 5W stays shinny to -13°F, while the 15W only stays skinny to 5°F.

If you let your engine warm up a bit before any driving, that's always a good thing, even in summer.

 

​ makes sense as to why cold start ups are better then. so with the full syn 15w40 going away, have a lot of guys gone to 5w40? i haven't really heard much about it.
and typically my truck runs 5-10 mins before I drive it (remote start) And usually let it cool off to below 350 on the egt or 5 mins. I want a turbo timer.

 

I could very well be wrong here, but I believe the recent trend for motor oils going to a lower cold temp rating - the W number - is for the newer engines which do have tighter tolerances which are affected by cold oil, hence the appearance of 10W-50 diesel engine oils, which ...

... if you're concerned about looser tolerances to due to wear, you might want to consider.

 

This is why I'm curious what other owners are doing since most trucks are getting up there in mileage. I can see how the 5w helps get oil in places on cold weather but that thicker 15w oil might be helping with loose tolerances.

 

Guess I could try it out for an interval and see how it feels. If I don't like it I will just stick with lucas or royal purple full syn

 

I guess I should have pointed this out - it's not 15w oil, it's 15W oil. The capitol W refers to "Winter", not weight. For multi-grade oils, say 15W-40, the 15W states that oil will flow like 15w oil at a temp of 5°F, yet when the oil temp goes to 212°F it's regular 40w oil. So once your engine is warmed up, the 15W part of the grade becomes irrelevant.

For some reason completely unknown to me, the operating temp viscosity of oil is measured at 212°F, while the Winter temp viscosity is measured at various temperatures. The lower the Winter viscosity, the lower the temperature at which that oil holds that viscosity.

You would think that oil gets thinner (viscosity decreases) as you increase the temp (and for straight grade oils it probably does), but for multi-grade oils they use viscosity additives which make oil thicker as the temp increases. So I think that 15W-40 is really something like 15w oil, and the additives make it go to 40w once it gets to 212°F. So what happens when these additives breakdown and become ineffective? Your 15W-40 becomes 15w oil all the time, which is far too skinny and your engine bearings hate you.

 

Several brief comments:

1) CJ-4 is the proper oil specification for our engines.

2) You can buy Motorcraft labelled 5w40 CJ-4 oil in multiple places.

3) If you have any leaks, the 5w40 may leak a little more before the oil gets up to temperature. Fix the leaks, if present, and enjoy the simplicity of using the CJ-4 rated 5w40 year round in both of your trucks.

 

After reading more threads I see some diesels have been recommended several weights such as 10w30 and with all your replies I am feeling more comfortable in varying from what has been the norm for me with the 7.3 engine. with the correctly rated 5w40 full syn I think things will work well.
also thanks for the explanation saint, I had learned this once upon a time now it's coming back to me.

I'm always pretty hesitant to change things on this truck because it drives and runs so well. Gets great mpg too so I'll be keeping an eye on that after the oil change.

 

I've used nothing but 5w40 full synthetic in my truck for 10 years now (over 175K miles of driving). HOWEVER, I just switched to a CJ-4 0w40 a couple months ago due to injector poppet wear (one injector doesn't fire until the oil viscosity drops as the engine gets up to temperature). I can honestly say that my cold startups are significantly improved with the 0w40, and I'm sticking with it until I get the injectors shimmed, and maybe even afterwards..