You shouldn't be tasting any oil from the roasting process. Many better quality teas have what we call a 'buttery' mouthfeel. It probably has something to do with the release of oils or other compounds in the tea. Comparing sencha with LJ is apples and oranges. Sencha by nature is more astringent with a different flavor profile. Steaming vs roasting(frying) produce very different teas. If unfamiliar with other teas besides sencha, I can see where one would have to get used to the taste. It took me a long time before I could accept sencha on to my palette. It's still not my favorite tea but I've grown to like it and drink it. LJ is my favorite green tea, btw. I love the oiliness and flavor profile.