I find that sweet potatoes cook unevenly - parts are firm, other parts are mushy. I bake at 350F for 45 minutes, no oil.
For Thanksgiving I sliced them length-wise and then cut the halves into half-moons. While the oven warmed to 400F, I sautéed the pieces in an oven-safe pan (I used butter, but oil is fine) for a few minutes and then let them finish in the oven for about 15-20 minutes, occasionally stirring them.
Uniform size gives you uniform cooking.