I think people worry about oil viscosity and oil mixing too much.  Think about it.  How many people don't even know what viscosity is let alone what kind of oil is in their engine.  Surely the goons in full service lanes in gas stations around the world have no clue what is all ready in there either when they do you the favor of "adding a quart" to your wifes car.

If mixing oils were that big of a problem today, I'm sure the world would be hearing round after round of oil catastrophe stories. which really don't seem to be happening on well maintained cars.  

That is not to say that you should be mixing in 20W-50 oil in your new engine during the winter in Fargo, North Dakota, but I think adding oil is like shooting a shotgun.  For most non-critical everyday applications, close is good enough as long as its the proper API service class or better and changed at the service interval written in your owners manual.  Severe weather and critical use conditions excepted.

Chumley  

I agree with Chumley.  Many of the new vehicles specify 5W-30 or even 5W-20, but most of the quick lube places and I'm not knocking them, and even the factory dealers have one big barrel of "oil" that is piped to each service bay, usually with 10W-30 and there aren't millions of seized up vehicles on the side of the road.  Will you achieve the EPA "window sticker" fuel mileage with 10W-30 in place of the recommended 5W-30?  Maybe not.  Will your engine wear out in 200,000 miles instead of 300,000?  Maybe, but if you use quality brands, how often you change the oil will make a bigger difference than being off 5 points on the viscosity.

Blacksmith

I agree that a little viscosity change one way or the other isn't going to make much difference, it's just that my 5.0L MPI T/A is 13 yrs old now and has 58K miles and has a little oil seepage around the valve guides.  My shop has suggested I move to a slightly thicker grade of oil from the Mobil 1 5w30 I currently use, especially in the summer months.

try 15w40 then
or even better stuff a 460 in there and keep your old motor to drop in for emmisions tests.

sciguyjim---If you have maintained your 5.0 as reccommended by the mfgr. and used Mobil 1 for the last 13 years---58K miles is just the BEGINNING of the life span of that engine, barring defects in mfgr.  Your problem most likely lies not in "seepage around the valve guides" but in hardened valve SEALS.  Try having a new set of seals installed (it can be done without removing the cyl heads) and see if that does not cure your 'problem'.  
In practice it is not really a problem unless it is causing plug fouling or excessive  oil consumption.  I personally don't get too excited unless consumption exceeds one quart between my usual oil changes and, even that is subject to question (as in a truck with a VERY large capacity sump).
Quite often in todays business world you find that additives and viscosity index 'improvers' are manufactured by chemical companies supplying the ENTIRE lubricant industry!  Mixing various brands and viscosities just is not normally a problem.  Just don't go to extremes as I once did and fill the sump with STP !!! (I was 'away' from home in a bugeye Sprite with a standard crank and 0.010 undersize bearings. I started the engine and drained  oil as I added STP.  Don't laugh, it got me home 900 miles away and NO damage!!!)


Rod

Thanks for your post, I'll look into having the seals replaced.

if you want to stop oil leakage past valve seals a higher viscosity does not always help. What matters in this case is much more the volatility of the baseoils used and not the viscosity grade. Synthetic 5W-20 or 5W-30 may be less volatile then a 15W-40

If you look at more recent US oil specs you will notice that nowadays in the more severe specs a socalled "NOACK"-volatility test is used - a test to establish the volatility originally developed in Europe and used there for decades. In practice this means that modern synthetic oils do meet these requirements where old fashioned lower spec oils might not.

Excellent suggestion to look for oils meeting the more stringent NOACK volatility tests.


A recent lab testing of leading 10W-40 synthetic oils showed a range of volatility on the NOACK test from 4.73% to almost 10% among the top 4 brands tested.  

Another NOACK test of top selling 15W-40 synthetic oils showed a volatility range of 6% to almost 14%.

Testing the top six synthetic 10W-30 oils showed volatility ranging from 6.7% to almost 18%!

As far as Viscosity Index (VI) improvers go, there is wide variety in the quality of VI improvers, and their ability to resist sludge formation over time. Additionally, there is a wide variety in the quantity of VI improvers used by various manufacturers.  

Recommendation:  Use a top-line synthetic oil with the lowest volatility and highest quality VI improvers (if any)from a manufacturer that has the most experience in synthetic oil technology.

Will
NOVA Engineering & Consulting
novaeng@tampabay.rr.com   

  

Wow, I didn't realize there was so much volatility from synthetics.  Thanks for the info.