The 6.6L Gas Engine (GM L8T) is a powerful V8 engine designed by General Motors for heavy-duty trucks like the Chevrolet Silverado 2500HD and 3500HD. Introduced in 2020, it replaced the older 6.0L Vortec engine and delivers strong towing capability, durability, and improved efficiency.
This engine is widely known for its cast-iron block strength, high torque output, and long service life when maintained with correct oil type and regular servicing.
In this guide, you will find complete details about engine specs, oil type, oil capacity, oil change interval, maintenance cost, and common issues.
6.6 Gas Engine Overview
The GM 6.6L L8T gasoline engine is engineered for heavy-duty performance. It uses a cast-iron block for maximum durability and is paired with modern fuel injection technology for better efficiency and power delivery.
It is commonly found in Chevrolet Silverado HD and GMC Sierra HD trucks, designed for towing, hauling, and commercial use.
Key Insight: The L8T 6.6L gas engine produces around 401 horsepower and 464 lb-ft torque, making it one of the strongest gasoline HD truck engines in its class.
Engine Specifications
Fuel Economy (MPG)
- City: 10–14 MPG
- Highway: 13–18 MPG
- Combined: 11–15 MPG
6.6 Gas Engine Oil Type
Using the correct oil is critical for engine life, especially in heavy-duty applications. GM recommends full synthetic oil that meets dexos1 specifications.
In colder climates, 0W-20 may improve cold starts, but 5W-30 remains the most commonly used viscosity for this engine.
Oil Capacity
- 6.6L Gas Engine: ~8 quarts (with oil filter)
- Always check dipstick after refill
- Do not overfill or underfill
Oil Change Interval
- Normal use: 5,000–7,500 miles
- Severe towing/heavy load: 3,000–5,000 miles
- Recommended: At least once per year
Oil Change Cost
- DIY: $60 – $110
- Workshop: $90 – $150
- Dealer service: $120 – $200
Common Problems
- Oil consumption in high-mileage engines
- Carbon buildup in direct injection system
- Slight cold-start roughness in winter
- Fuel economy variation under heavy towing
- Sensor-related warning lights (rare)
Maintenance Tips
- Use only dexos-approved synthetic oil
- Replace oil filter every oil change
- Check oil level every 1,000 miles
- Avoid extended high-RPM driving when cold
- Service air filter regularly for better efficiency
Frequently Asked Questions (FAQ)
What oil does the 6.6 gas engine take?
The recommended oil is SAE 5W-30 full synthetic that meets GM dexos1 specifications.
How much oil does a 6.6 gas engine hold?
It holds approximately 8 quarts including the oil filter.
How often should oil be changed?
Every 5,000–7,500 miles depending on driving conditions.
Is the 6.6 gas engine reliable?
Yes, it is a durable heavy-duty engine when maintained with proper oil and service intervals.
Final Thoughts
The GM 6.6L L8T gas engine is a strong and reliable powertrain designed for heavy-duty work. With proper oil maintenance and regular servicing, it can deliver long-lasting performance for towing and commercial applications.
- Engine: 6.6L V8 Gas (L8T)
- Power: 401 HP
- Torque: 464 lb-ft
- Oil Type: 5W-30 Full Synthetic
- Oil Capacity: 8 Quarts
- Oil Change: 5,000–7,500 miles