Lets do a little Automotive 101 together shall we?
Oil perfroms several functions within the engine. It lubricates, cools, flushes away debris, carries this debris to your oil filter and protects the moving parts within your engine.
You may have encountered the statement that the oil does not wear out. That statement is only partly true. The base stock, (the actual oil), does not wear out. However, the detergent packages that are engineered with the oil to improve its lubricating and cleaning properties does. This is the reason for frequent oil changes.
The rule of thumb is every 3000 miles or 90 days whichever occurs first. This ensures that the additive packages in the oil are not overloaded and unable to perform their tasks of cleaning and lubricating the moving parts.
Failure to change engine oil is a major contributor to engine sludge buildup, excessive wear and heavy oil consuption directly related to this engine wear.
As for your vehicle, the person who told you to use Straight Weight Oils rather than the Multi-Viscosity Oils is incorrect. Straight weight oils produce more engine wear on cold start-up than the Multi-Grade Oils. This is very important because we are now coming out of the summer months and entering into the colder months of the year.
On your Ford Ranger, (depending on milage), I would reccommend that you use 20w-50 weight oil in that engine from now on until you sell or retire the vehicle.
Here is a guide that you can use. We use this guide in the field to determine what oil to use with what vehicle.
New Vehicle, (0 to 30,000 Miles):
Intermediate Vehicle, (30,001 to 70,000 Miles):
10w-30, 10w-40, 15w-40
High Milage Vehicle, (70,001 to 100,000+ Miles):
15w-40, 15w-50, 20w-50
There is a good chance that the oil was contributing to the poor fuel milage, but not to the point at which you described. This is normally a sign that the engine may be due for a good tune up, and general service.
This may include all of the following:
Engine Oil and Oil Filter
Cooing System Service
Transmission Fluid Service
Fuel Injector Service
Rear Differential Service
This would be considered a major tune up on your truck, and if it has not been done since you owned it, I would highly advise that it be done along with a complete diagnostic to ensure all of the other systems are functioning properly.
Poor fuel economy can also be attributed to:
Low Tire Pressure
Incorrect wheel alignment
Malfunctioning Emissions Equipment
Malfunctioning Fuel Management Systems
Excessive Engine Wear
This is why I made the reccommendation to have the vehicle diagnosed in a reputable shop or your local Ford Dealership's Service Department.
Yes contrary to what you may have been told, Ford will still service this truck for you. The Dealerships are not just for the newest cars and trucks on the road. They will service this truck for you, and probably be able to provide you with a more complete diagnostic as to what all of the issues are facing this vehicle, and whether its worth you time to keep the truck, sell it or trade up for a new one.
Of course if you are going to keep it then by all means have them service the truck for you. Remember the Dealerships are going to be more expensive to have your vehicle serviced, but the service they provide is normally better, the environment in which you are greeted is normally clean and friendly. Dealerships also maintian the most current and up-to-date service information for your vehicle. Ford is just like GM in this area. All of the Dealership Service Departments are constantly updated with factory service information. So if there are any warranty recall, (campaign) items that need to be addressed, they can and will handle it right there.
These business also keep record of your service intervals, and the services and/or repairs that have been made to your vehicle. This is very important if a part failure shoudl occur, they can cover it for you with very little hassle.
Lastly. Once you have started with a particular brand of oil, you need to stick with that brand of oil. Switching brands to suit what is on sale is an excellent recipie for engine sludge build-up.
I hope this helped you out. Good Luck!
General Motors Service Parts Pro