Originally Posted By: SecondMonkey
I have a rather large collection of various B&S engines...I like to use 40 weight oils in the older ones, but I'm wondering why the Briggs factory recommendation has always been 30w? What is the disadvantage to a 40w oil?
Most engines are designed to operate with SAE-30 oil as the best compromise between being able to start cold, circulate, and protect while at operating temperatures. Multi-grade oils improve the starting and circulation capability, but sacrifice some protection at operating temperatures.
That being said, most small engines are splash lubricated rather than pumped circulation. 10wt, 20wt, 30wt, 40wt in a splash system are all going to "circulate" the same since the oil's being thrown around by a dipper on the end of the connecting rod or crankshaft.
The thicker the oil, the better it's going to protect moving metal parts. So, there really is no disadvantage to using SAE-40 oil in your small engines other than a slight increase in fuel consumption to overcome the added viscosity drag in the bearings and between the piston and the cylinder. You'll never notice that difference.