When it comes to selecting engine oil for heavy duty diesel trucks, one of the most common questions fleet managers faces is whether to use conventional or synthetic oil. Both offer distinct advantages and disadvantages. Here is an in-depth look at the key considerations in the synthetic versus conventional oil decision for heavy duty engines.
Formulation Differences
Conventional engine oils are refined from crude oil, with additives added to enhance certain properties. The molecules in conventional oil vary in size and shape. Synthetic oils are chemically engineered to produce uniform molecules designed for specific performance traits. The manmade structure of synthetics allows tailoring properties like volatility, oxidation resistance, and flow characteristics.
Viscosity and Operating Temperatures
A key advantage of synthetics is viscosity index – how much an oil's thickness changes with temperature. Synthetics have higher viscosity indexes, resisting thinning at high temps and thickening in cold better than conventional. This allows improved protection across a wide temperature range. Synthetics also have lower pour points for better cold weather flow.
Resistance to Oxidation and Breakdown
The uniform molecules in synthetics are less reactive and slow the formation of sludge and deposits. Synthetics better resist viscosity breakdown from heat and contamination. Conventional oils are more prone to thickening, deposit formation, and performance deterioration over time and mileage.
Volatility and Oil Consumption
Synthetics have lower volatility, meaning less evaporation and oil burn-off at high engine temps. This reduces oil consumption and keeps oil where it is needed – lubricating the engine. Less evaporation also means reduced oil top-offs between drains.
Engine Wear Protection
Both synthetic and conventional oils must meet API certification standards for wear prevention, deposits, etc. But some synthetics excel in anti-wear properties due to additive packages and inherent lubricity of synthetic base oils. They provide a margin of extra protection for high mileage, high load applications.
Extending Drain Intervals
The advanced properties of synthetics allow significantly extended oil drain intervals, often doubling or tripling conventional oil change miles. Less oxidation and viscosity breakdown over time enables pushing mileage limits. This can reduce downtime for maintenance. Oil analysis helps determine optimal drain intervals.
Cold Weather Performance
Synthetics flow better than conventional oils in sub-zero temperatures. This allows easier cold starting and faster lubrication at start-up. Conventional oils can leave critical components unprotected until adequate warm up. Synthetics prevent wear in cold climates.
Fuel Economy
Reduced friction from synthetic lubrication can provide incremental fuel economy benefits. Industry estimates range from 0.5-2% MPG improvement with synthetics. Over hundreds of thousands of miles, the fuel savings add up.
Cost Difference
The biggest trade-off is higher cost. Sinopec told us that synthetics can cost 5x as much as conventional oils. This makes the initial price tag hard to justify. But when factoring extended drain intervals and long term engine life, synthetics become more cost competitive.
New Engine Break-In
For new or rebuilt engines, conventional are typically recommended during initial break-in periods before switching to synthetics. The uniform molecule size of synthetics may prevent proper mating of new engine surfaces.
Compatibility Concerns
If converting an older engine from conventional oil, flushing the system first is advised. Deposits shed by old oil can react poorly with synthetics. But overall, synthetics and conventional are compatible and miscible.
Manufacturers Specs
OEM recommendations should guide oil selection. Many approve synthetics due to increased durability requirements of modern diesel engines. Look for oils meeting API FA-4 or CK-4 specs as newer heavy duty oil categories.
Oil Change Best Practices
Beyond selecting the right oil, proper maintenance practices during oil changes are equally important for heavy duty diesel engine longevity. Here are some best practices:
- Follow OEM Guidelines - Adhere to manufacturer recommendations for oil type, viscosity, change intervals, filter selection, and any special procedures. This ensures optimal protection.
- Choose High Quality Filters - Premium filters with high contaminant holding capacity maintain filtration efficiency over service life. Use brand name filters meeting OEM specifications.
- Tighten to Spec - Under or overtightening the oil filter can lead to leaks. Tighten dripless filters by hand until gasket contact, then rotate an additional 3/4 to 1 full turn based on specifications.
- Check Used Oil - Inspect used oil for coolant mixing, metal particles, or other signs of abnormal wear. Change intervals may need adjustment if issues are spotted.
- Top Off Levels - Before adding new oil, fill to proper level on dipstick according to manufacturer specifications, usually around engine oil fill neck.
- Follow Drain Intervals - Extending intervals beyond OEM guidelines leaves engines unprotected. Synthetics allow longer intervals but still respect limits.
- Use Correct Tools - Drain plug washers and filter gaskets rely on proper torque specs. Use calibrated torque wrenches to tighten components.
- Examine Drain Plug - Replace drain plug washers if worn or damaged. This prevents leaks and ensures proper torque specs can be achieved.
- Dispose Properly - Follow regulations when disposing of used motor oil, which contains heavy metals and other contaminants requiring special handling.
By adhering to these best practices during oil changes, trucks will operate reliably at peak efficiency. Attention to detail prevents overlooked steps that could compromise engine protection down the road.
Final words
For most heavy-duty trucking applications, the extra protection and extended drain intervals of synthetic oils justify the higher costs. Conventional oils still have a place for budget conscious fleets doing regular maintenance. Work with your OEM and oil supplier to determine the optimal conventional or synthetic oil choice based on engine design, climate, routes, and economics.
While no oil can substitute for diligent maintenance practices, using premium synthetics gives diesel engines the best chance for long mileage, minimal wear, and sustained reliability. Consult oil analysis reports and continue tweaking type and drain intervals to optimize costs and engine life.