I have a 99 Ranger with 123,000 miles on it. 4.0L motor. I put Mobil1 5W-30 in it recently and it has been losing oil. Just had the valve cover gaskets done. I was told that it might have bad valve seals or weak rings. So, I bought 10W-40. Will this stop the oil from burning? Or should I use 15-40, 15-50, or 20-50? I don't have the money to have the heads removed and new parts put in.