Flush engine oil when changing oil viscosity?

Messages
3,580
Location
St. Charles County, Missouri
Thank you for the answers, really helpful to see so many quality opinions.
I was thinking of a "flush" as the liquid you pour in the engine to clean it from sludge or whatever else, because maybe the viscosities were incompatible so I'd need to get rid of all the 5w-30 ... turns out there is no such need.
Will also definitely clean the PCV as Astro14 mentioned!
Viscoscities aren't incompatable, although I remember calling the dealer panicked when I poured a quart of 10w40 0n top of some 10w30 in a Fiat 600 back in 1966. Actually thought it would stratify in the crankcase. Look all I've learned in 54 years and 2 months.
 

4WD

Messages
14,394
Location
Texas
I'm sorry, what? You are actually advocating running an engine with the drain plug out?
Sure hope not …
Well, had one quart of MSS 5w20 leftover … that went in with the plug out … displaced old oil/break in metal …
(then filled with fresh oil)
Plus I was about to re run the XG filter, offset that a bit …
 
Messages
8,806
Location
Virginia
I don't know... Only time I'd run a motor with no oil in it is when I truly did not care if it blew up or not.

There's a video I have seen where a guy pulled the oil pan cover off and started the car... Then proceeded to rev the car up a lot... Until it finally blew up. Was fun to watch... :LOL:

Kinda like the other thread where I put in the movie Cobra...

Where in the movie Cobra aka Stallone's character says to the crazy guy with a bomb says he's going to blow up the store they are in and Stallone's character Cobra says... "Go ahead, I don't shop here.". :LOL:
 
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Messages
5,716
Location
Baltimore, Maryland, USA
Flush engine oil when changing oil viscosity?


 
Messages
1,125
Location
Berks County/Pa.
I don't know... Only time I'd run a motor with no oil in it is when I truly did not care if it blew up or not.

There's a video I have seen where a guy pulled the oil pan cover off and started the car... Then proceeded to rev the car up a lot... Until it finally blew up. Was fun to watch... :LOL:

Kinda like the other thread where I put in the movie Cobra...

Where in the movie Cobra aka Stallone's character says to the crazy guy with a bomb says he's going to blow up the store they are in and Stallone's character Cobra says... "Go ahead, I don't shop here.". :LOL:
You wanna go to Hell with me pig -- you wanna go to hell with me pig!! Lol
 
Messages
1,329
Location
California
I assume the OP is concerned about either mixing viscosities or mixing questionable oil with new oil.
About mixing viscosities... oil viscosity is not a static thing, it does not stay the same. Between fuel dilution, shear, suspension of contaminants (dispersant effects), oil viscosity does change slightly during a service interval. So it doesn't matter if a little of the previous viscosity mixes with the new. it will be at least 90% new oil so don't worry about it.
About mixing questionable oil with new... again it will be 90% new oil so whatever was in there before will be basically "consumed" by the large quantity of new oil.
So, just change the oil and enjoy the car. No need for a flush or any other worry.
 
Messages
491
Location
GA
Then how are you going to get the engine flush out? 😁 You're going to have residual fluid left. Better to be oil than a solvent.
 
Messages
894
Maybe the OP is misleading. It's not a "I installed 5w30, ran that for 10min, but now want 0w16". It's more like "I have this old used engine oil in the engine, should I flush it out?".

A lot of old oil will come to the pan when the engine idles for about 10sec w/ drain plug out.

Don't idle the engine with the drain plug out, Mr. Russels.
 
Messages
3
Location
Glenmont, NY (near Albany)
Thank you for the answers, really helpful to see so many quality opinions.
I was thinking of a "flush" as the liquid you pour in the engine to clean it from sludge or whatever else, because maybe the viscosities were incompatible so I'd need to get rid of all the 5w-30 ... turns out there is no such need.
Will also definitely clean the PCV as Astro14 mentioned!
Actually, I would much rather have the 50 or so ml of plain used oil staying behind than 50 ml of used oil contaminated with flush.

I don't know what standards prevail in Europe, but API requirements are reflected in this answer by Valvoline:
Motor oil manufacturers must meet minimum industry standards for performance and compatibility of the API mark. However, make sure you follow the recommendations provided in your owner's manual for viscosity and API category.
 
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