oil cause unholy gas mileage?

[azn2nr]

i changed my oil to 10w 30 about 2000 miles ago before the coldest winter on the books for ten years came around but for the last few weeks i loaned my car to a friend who had his in the shop. a week before he gave it back he told me it was sucking gas like none other and now that i have it back the mileage has gone to 12mpgs from about 21mpgs. is it posible that the warmer weather and thicker oil is causing the problem???

[DLC]

I'm going back to 15w40 on my next oil change. Oil viscosity has very, very little to do with gas mileage. Drive it another tank and see what it gets.