In my chainsaw (Homelite), the season of the year wouldn't really effect the
weight of the oil. The oil tank is part of the alum engine casting. In cold
weather, the oil regular bar oil is difficult to pump, however only a minute
of engine operation has the oil warmed up and I'm then able to pump it on to
the chain guide.

I think 30 weight would be too light and wouldn't cling to the bar and
chain. I have tired it in a pinch and it seemed like I would have to pump
oil continiously while sawing.

I am a frugal guy but I won't skimp on the bar oil, knowing what a
replacement bar costs. They wear down fast enough as it is.

--
My opinion and experience. FWIW

Steve