Why the thick stuff.

I think 15W50 is too thick for a typical Ford small block.  They are supposed to run 10W30. 

Oil isn't a place I look to save money.  I operate under the "oil is cheap, engine's expensive" banner.  I do get the whole "its more important that you change your oil than what oil you use" thinking, but that's also assuming the oil is the correct weight. 

ZDDP can be had pretty cheap from the commonly available STP additive available in most auto parts stores.  I drop a bottle in my oil change and just run conventional 5W30 from NAPA changed once a year at the start of the driving season. 

If you want to run synthetic without another additive there are numerous oils from Redline, Amsoil, Motul, etc. that all have adequate ZDDP already in them, and can be had in 10W30 or 5W30 weights. 

Its not about breaking the cam in, or if the cam's already broken in.  The issue is the area of high load and potential wear created where the lifter interfaces with the cam lobe, where ZDDP provides protection.  Those areas always exist in a flat tappet engine.

Now, is the 800ppm currently in oils really the issue its been made out to be?  Is it like the loss of lead in the fuel?  I can't say, but I can say I'm not risking my flat tappet engine.  I only have one left.  Everything else is OHC or roller tappet. 

Might wanna 'cut' that 15-40 with a quart of Dexron ATF!!   J/K
6s6

Back in the days of the flat head we used single weight oil, we always added a quart of Rislone.
Can’t remember when dual viscosity oil was introduced.

Up here in Michigan where it is always toasty warm, I have run 20W50 for years. Never lost a cam or bearing but I do two things as a rule. Open up bearing clearances a half a thousandth or more and run a 195 thermostat at the least. Always try to get 210 degrees running temp while DRIVING! Oh! it is Great! What I have found is that more bearing clearance allows engine to rev free-er and the 20W50 keeps oil pressure right where I like it. I worry a lot.

 Yrs ago as a kid, I thought it was a good idea to use 50W in my 65. The first cold morning I heard that pump shaft twist and snap on startup.

I build loose engines since all the dyno testing shows more HP with another thou of clearance even with thicker oil. Now you gotta realize I am usually building race motors so they make peak power at more revs than most street engines, except mine. We also found that more HP came with another 20 degrees or so of water temp. So hot running loose motors like thicker oil. All my newer street machines run what ever the owners manual says.

6sally6 wrote:

Bearing Bob wrote:

So if I use 15w50 in my motor, what is the viscosity at start up?

Well................'they-say'....its 15 when cold and 50 when hot.     I don't believ'em.......what do they know!?!!
6sal6
 

 
It’s magic Mike with a little bit of special polymer additive.

TKOPerformance wrote:

Oil viscosity is a logarithmic function respective of temperature.  The slope of that function is lessened with multi viscosity oils.  The idea is to get the cold pourability of the lighter weight oil and the protection when hot of the heavier oil.  Typically this is done by adding viscosity index improvers to the oil, though not always (synthetics often don't need VIIs).  Oil weights are assigned based on the oil's performance on the SAE J300 test.  A multi grade oil passes that test for both its numeric grades. 

But, viscosity and oil weight aren't the same thing.  Viscosity is kind of one of those things we talk around because it isn't fully explained or understood.  Oil weight is given as a number like "40" or "10W30".  Viscosity is measured in cSt (centiStokes), which is 1/100th of a Stoke (St, the "S' is capitalized because its a unit named for George Stokes, a mathematician who worked with fluid mechanics), or 1 cm/s^2.  The test that creates the oil's rating in cSt measures how far the oil flows in centimeters/second on an angled testing apparatus at a specified temperature. 

I only mention this because if you search for "oil viscosity at temperature", or some permutation thereof, you aren't going to find a nice graph or chart that says "at 32 degrees F 15W50 oil is a 15 weight oil".  Instead you'll see that at 32 degrees F 15W50 oil has a viscosity of 2,500 cSt.  Since it doesn't directly correlate to oil weight it's only important as it pertains to comparing it to another oil.  So the real thing to look at is viscosity in cSt at a given temperature for say 15W50 vs. 10W40 (which is only 350 cSt at 32 degrees F). 

In summation, that 15W50 flows a lot worse than the 10W30 the engine should have in it at 32 degrees (like 10 times worse).  Its a slope though, and the higher the weight the steeper the slope tends to be with a multi grade oil.  This therefore really only applies to cold temperatures.  By the time the engine is up to operating temperature the difference at say 200 degrees is almost nothing.  Where you really have to be careful is cold starts, especially considering that most engine wear occurs at start up. 

 I knew that but can only type with two fingers, it would have taken me into April to post it.
Guess I should get voice to text software!