Sketching in watercolours on site (plein air) is the usual way to go for most artists, since equipment is light and technique fast.; but if you are painting on a day trip, oils are often the best way to go, especially in inclement weather. There is nothing more devastating than rain to a watercolour, while an oil is completely impervious to it. Oil sketches are also more likely to be treated as serious art. Witness, for example, the incredibly lively oils of the Group of Seven. The 8 x10 inch paintings of Tom Thompson express a liveliness that his larger works miss. These were all done on the spot. Many of the French Impressionists rarely painted indoors and produced huge canvases en plein air. Monet, Pissaro, Sisley and, of course Van Gogh relished their time in the sun.
On North Vancouver Island, when I head out for a day of painting I nearly always use oils, even on a sunny day but especially on a showery one. I have painted wearing rain gear and watching the water running down my canvas while I worked! My usual equipment is a French easel, often accompanied by a stool for comfort. The only drawback to using oils is their slow drying time, which means being very careful getting the finished product home! I love to use a slow drying white called Zinc White(because of its transparency it doesn't hide the pigments that are mixed with it) so my oils take a very long time to dry.
Above is a photo of my easel on a newly logged area high above Nimpkish Lake on a cloudless day early this summer.