That is a bit overkill and 10W40 is actually too heavy for most climate conditions. I recommend you use what the manufacturer suggests. Most require 10W30 and change at regular intervals.
Interesting! In Australia - Up until probably the last 10 years, 10W/40, 15W/40, even 20W/40 was probably the most popular mineral multigrade in both petrol & diesel engines. Most small air-cooled engines used a straight (mono grade) W30. They all seemed to do the job.
It's all changed now with the advent of semi & full synthetic oils. Probably the most common diesel/petrol oils are 5W & 10W/30's semi & full synthetic. Small air-cooled engines have also changed, with many service/sales shops recommending multigrades in 5W & 10W/30's semi & full synthetic. Some stores no longer stocking the single grade W30.
I think it likly that the change to lower weight oils has more to do with meeting emission standards, than any benefit to the machines service life. In general I would think a ?W/40 would be a better bet, than a ?W/30, for a hard working engine (commercial) in high ambient temperatures.
For small air cooled engines, the change from mono grade (W30) to multigrade synthetic, may be of benefit to commercially operated small air-cooled engines but is unlikly to be cost effective for the average homeowner. The problem is finding the mono, now that so many suppliers have moved to multigrade semi & full synthetics for these engines.
As for 10W40 V 10W30
I stand to be corrected: -
My understanding is that both these oils will have the same cold viscosity (W10) and only as the engine heats the oil, will the viscosity change to that of a 40 or 30 weight.
The W10 will facilitate faster circulation and less resistance to cranking (easier starting)
In theory, viscosity is an indicator of the ability of the oil to maintain component separation. In monograde oils the viscosity lessens (oil gets thinner) as it is heated, reducing is ability to maintain component separation. In multigrades the opposite occurs - as the oil is heated it starts to exhibit the attributes of a higher viscosity oil. Therefore the W30 will be more applicable to cooler climates and the W40 to hotter.
It is unlikly that the operator would see any difference, in engine performance, between these two multigrade oils.
