The 5w30 oil is better than 10w40 oil at lower temperatures as it's thinner and will flow better when it's colder out.
The 10w40 oil is better at higher temperatures and will flow better at hotter temperatures.
The difference between 10w 40 and 10w 30 is the viscosity.
The 10w 40 oil is thicker than 10w 30 at high temperatures while the 10w 30 oil is thinner at high temperatures.
The meaning of 10W40 is the thickness.
The W in in 10W40 means winter which signifies the viscosity of the oil for low temperatures.
The number 40 indicates the viscosity of the oil at normal operating temperatures when the engine is running.
The 10W-40 oil which is a high viscosity oil can be useful for older engines burning or leaking oil.
10W-40 high viscosity motor oils for use in vehicles with more than 75,000 miles, and a synthetic blend oil when cost is a consideration.
The difference between 10W-30 and a 10W-40 is the high temperature viscosity.
Obviously, a 10W-40 is thicker than a 10W-30 at high temperature.
You won't likely see 10W-40 as an oil recommendation on a modern-day passenger car.
However, it still retains popularity with medium and heavy-duty gasoline engines in light trucks.
This oil weight is commonly used in diesel engines or in a smaller motorcycle engine too.
It's best to use the oil viscosity recommended in the owner's manual, but using a slightly thinner or thicker oil (lighter or heavier viscosity) generally poses no long-term harm.
The difference between 10w30 and 10w40 oil is the thickness of the engine oil at engine operating temperatures.
The 10w40 oil is thicker at engine operating temperatures and the 10w30 engine oil is thinner while at engine operating temperatures.
So in simple terms the difference between 10w40 and 10w30 is 10w40 is thicker while 10w30 is thinner oil.