Can I use in a motor saw 15w40(in a reservoir dedicated for chain oil) instead of sae30(chain oil). Sorry for this question . but I have plenty oil(15w40) and I can't use on my car( because it requires 5w40) so can I use in motor saw? Thanks
The SAE numbers were "monograde" specifications where the change in viscosity with temperature was not described explicitly. Higher performance oils for car engines were "multigrade" and the cold and hot viscosities were specified separately.
A 15w40 oil is equivalent to SAE15 when it is cold, and to SAE40 when it is hot.
So your 15W40 will be too thin when it is cold, and too thick when it is hot, compared with the specified SAE30. "Hot" means an oil temperature of about 100C, and presumably your chain oil never gets anywhere near that temperature.
There isn't a simple linear relation between the numbers in the SAE specification and the viscosity - they don't mean that a grade 30 oil is "twice as thick" as a grade 15. In fact, SAE had to tweak their original numbering scheme for low numbered grades as thinner oils became standard, to avoid having grades with negative numbers!
Note, I have no idea how critical the specification of chain oil is, so you are on your own in deciding if all this is important or not.