Can oil "expand"?

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2,444
Location
Indiana
I apologize if it's a stupid question. I ask this because I just took a 6 hour trip recently averaging between 80-90 mph (3200-4000 rpms) and afterwards, my oil level was about 1/8 of inch higher on the dipstick than before the trip. Whats more, since that trip, my gas mileage *increased* noticeably. For example, my commute to work is about 18 miles and I averaged about 30 mpg before the trip and now I average about 31.5 mpg. The increase in gas mileage is not completely surprising considering that my engine is still probably breaking in with about 8000 miles on it. I only mention it because I observed a noticeable jump after the rise in oil level. So the questions I have are: 1. Could my oil have thinned and "expanded" causing it to sit higher on the dipstick and causing the gain in gas mileage? I'm using Castrol GTX 5w-30 2. Could I have a coolant leak? I don't think I have a coolant leak because the coolant level is stable and my oil is crystal clear (I heard it is suppose to darken if coolant is getting into the oil) with about 1200 miles on it. But I'm open to other thoughts about this. Oh yeah, one more thing, I noticed that my oil level sat a little higher after about 600 miles after my last oil change too. Any gurus have insights for me? Thanks.
 
Messages
11,006
Location
Canberra ACT Australia
Veedub I have Redline 75W90 NS in my Outback manual trans and the level is much higher when hot. I queried Subaru on when to check the level, hot or cold and cold they said as the level will be higher hot. Hope that helps?
 

VeeDubb

Thread starter
Messages
2,444
Location
Indiana
quote:
Originally posted by sprintman: Veedub I have Redline 75W90 NS in my Outback manual trans and the level is much higher when hot. I queried Subaru on when to check the level, hot or cold and cold they said as the level will be higher hot. Hope that helps?
Sprintman, thanks for the suggestion. The problem is that the oil is generally higher on the dipstick when cold. However, after the trip, the oil was noticeably higher on the dipstick, warm or cold. In fact, the new warm level after the trip is as high as the old cold level before the trip (and yes, I did check it on the same level driveway, on the same spot even). It's a mystery to me because my gas mileage seemed to have shot up since the increase in oil level. By the way, if coolant is getting into the engine, what are some signs of this? If there is no discolored oil, or smoke coming out the exhaust, can I be safe in assuming that I don't have a coolant leak?
 
Messages
3,346
Location
Clarksville, Tennessee
Yes, As oil heats it expandes. I notice this most on my BMW Motorcycle, when I park it hot the level is above the mark that I keep it at. Another reason to check your oil cold on level ground, you will then never be overfilled or underfilled.
 

MolaKule

Staff member
Messages
21,599
Location
Iowegia - USA
Oil increase it volume (thermal expansion) as it heats up. The only sure fire way to check for coolant in oil is to have a used oil analysis done. If the coolant level drops and the oil level increases "AT the same time" then you may have a coolant leak. Coolant in the oil is observed as a milky like substance, but by the time you observe this, you may have spun a bearing or at least corroded a bearing.
 

Patman

Staff member
Messages
21,989
Location
Oakville, Ontario
I'm confused though. If oil increases in volume as it heats up, how come if I check my oil hot it shows the same level as if I check it cold?
 
Messages
485
Location
Montgomery, AL
You will see a drop in coolant if you have a coolant or head gasket leak. It be level for a while then one day be down. It may not slowly go down in a regular fashion. My coolant leak resulted in dark fluid and made it thicker. I noticed a "bubble" of oil on the dipstick instead of the usual thin layer of oil. No milkiness whatsoever. A mechanic can do a pressure test with a small hand held guage, very quick, or you can have your oil analyzed for sodium.
 
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