Rick's Free Auto Repair Advice

Thicker oil

Should you use thicker oil?

Lots of forum users ask if they should use a thicker oil in warmer climates or use thicker oil to fix low oil pressure. The answer is NO. Engine designers specify a certain oil viscosity for a reason and it’s based on bearing clearances, engine cooling needs, oil pump capacity, fuel pump needs, and valve timing mechanisms. The second you change to a thicker oil to fix low oil pressure you impact the operation of the engine.

What happens when you use a thicker oil

Let’s say your engine requires 5W-30 and you want to substitute 10W-40 oil because you live in a warm climate or want to fix low oil pressure issues. If you live in a warm climate, the 10W versus 5W rating won’t make much difference, so the only question is; will the 40 weight oil makes a difference over the 30 weight oil?

The reasoning behind the question is that you presumably live in a climate where the average daily temperature is close to 100°F. But engines run in the 200°F -215°F range, so the fact that the outside temperature is close to 100°F is irrelevant to the engine. Once the engine is up to operating temperature, the thicker oil won’t flow as well as 30-weight oil. It’ll create more friction and heat which causes slightly more engine wear. So no, it’s not a good idea.

Now, if that same vehicle is used in a colder climate, the difference between 5W and 10W is enough to make a difference in winter starting. In cold weather, the 10W oil will be thicker than the 5W and can slow engine cranking to the point where it may not start.

Thicker oil and friction

All oil thins when hot. A 10W-40 oil will thin less than a 5W-30. Sounds good, right? But thicker 40-weight, by its very nature, has more internal friction, so it won’t flow quickly enough to fill the spaces between bearings (bearing clearances). In other words, thicker 40-weight oil can’t create a consistent lubricating film in an engine designed for 30-weight oil. Worse than that, the reduced flow and higher internal friction mean the oil can’t remove heat, which is one of its main jobs. So metal parts will run hotter and wear faster, making your already serious wear problem even worse.

• Motor oil is the primary cooling agent for pistons

thicker oil

Worn piston

and rings. Since thicker oil reduces flow and increases piston and ring temperatures,  you actually accelerate oil breakdown and deposits on the ring land areas. That can cause the rings to stick and even weld to the piston.

Third, thicker oil has more resistance, so it creates more friction and heat–the LAST thing you want in an engine that’s already under stress. That extra friction also reduces gas mileage.

Fourth, thicker oil affects variable valve timing mechanisms. VVT timing is accomplished by pulsing oil pressure to valves to advance or retard timing. If the oil doesn’t flow as well, the VVT mechanism can’t work properly. So using the wrong oil can actually cause a check engine light.

Summary of reasons not to use thicker oil to fix low oil pressure

• thicker oil generates more heat due to additional friction, resulting in excessive oil oxidation sludge and varnish buildup

• thicker oil creates cavitation–the formation of air bubbles in the low-pressure area of the pump. Cavitation damages the surfaces in the pump and introduces air bubbles to the oil. Air bubbles prevent adequate oil flow to bearings and act as an insulator, further preventing proper oil cooling.

• thicker oil generates oil “whip” in journal bearings

• thicker oil requires more energy to pump

• thicker oil makes cold starting even more difficult

Understand Hydrodynamic Lubrication (HL) and Boundary Lubrication (BL)

The rod, crankshaft, and camshaft bearings aren’t sealed. The oil pump supplies pressurized oil to the bearing and that same oil constantly squirts out the sides of the bearings as the crankshaft or camshaft spin. As long as the oil pump can supply fresh oil at the right pressure at a faster rate than the oil leaks out, you’ll have Hydrodynamic Lubrication. In other words, the crankshaft, camshaft, rods, rockers, pushrods, etc will operate on a cushion of pressurized oil.

However, if you’ve neglected maintenance and the bearing surfaces wear to the point where the oil pump can’t pump enough oil at the right pressure to maintain oil pressure in those clearances, you wind up with Boundary Lubrication. At that point, the oil’s anti-wear additives are the only thing preventing metal-to-metal destruction. In older oil formulations, Zinc dithiophosphates (ZDP) or zinc dialkyl dithiophosphates (ZDDP) were used as the anti-wear additives. When the engine loses HL, and metal-to-metal contact begins, the ZDP and ZDDP additives melt out of suspension and act as a sacrificial lubricant. Lose of HL can happen on a cold start before the oil has a chance to build volume and pressure, or on a warm engine that has excessive bearing clearances and can’t maintain proper oil pressure. Those are only times these additives come into play. The irony is that in a neglected vehicle, those anti-wear additives are likely already used up due to the extended drain intervals.

Borate anti-wear compounds are sometimes used in newer oil formulations in conjunction with lower doses of ZDP and ZDDP. But the newest oil formulations use Molybdenum disulfide (MoS2). The advantage of MoS2 is that it actually plates onto the bearing surfaces to provide a slippery surface. In addition, MoS2 reduces friction and increases fuel efficiency. So oil manufacturers use it to provide anti-wear, protect the oil against the effects of extreme pressure, serves as an anti-oxidant, and a friction modifier. The downside, of course, is the higher cost.

Aftermarket oil additives/supplements

The auto parts store shelves are loaded with “miracle” oil additives. Many contain chlorinated paraffins. They serve as extreme pressure additives and friction modifiers. They do work, but can also corrode some engine metals, so they are NOT approved by engine manufacturers.

Polytetrafluoroethylene or PTFE “Teflon” is also used in aftermarket supplements. PTFE is suspended as a solid in the oil and claims to reduce friction. PTFE isn’t approved by any car makers and Dupont will not sell PTFE for use as an oil additive.

The additives that claim to reduce oil burning, sludge, and engine noise use a variety of solvents, detergents, or viscosity improvers. Do they work? Sometimes. But seriously, you’re better off using a higher quality synthetic oil than using these miracle additives.

Other engine oil additives

  • Corrosion additives slow engine corrosion, oxidation, and rust. The most common are Calcium and Barium to neutralize acids.
  • Anti-foam agents. As the crankshaft spins it can whip air into the oil, creating foam. Foam doesn’t pump well and it acts as an insulating factor instead of a heat dissipating agent.
  • Dispersant additives. These additives keep dirt contaminants suspended in the oil so they can be filtered out by the oil filter. Without suspension agents, the dirt contaminants would settle to the bottom of the oil pan.
  • Detergent additives. These cleaners dissolve sludge and act to prevent new sludge formations and neutralize acids.
  • Viscosity Improvers. Oil tends to thin as it warms. Viscosity improvers are either think straight polymers or polymers that “unwind” and straighten out at higher temperatures. They help maintain viscosity over a wider temperature range. However, these polymers have a weak spot; they are fragile and shear easily. So the oil loses its ability to maintain stable viscosity as it’s used.

©, 2016 Rick Muscoplat

Posted on by Rick Muscoplat

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