What Kind Of Oil Do I Put In My 91 Toyota Pickup. 10W-30 or 10W-40?

Depends on the season and temperature that it will be driven in.
Generally 10w-40 is for the summer, and the thinner 10w-30 is for the winter, if you so desire.

According to the chart, 5w-30 is preferred but 10w-30 can be used in 0 deg. F up to 100 deg. F and beyond.

Fuel and Engine Oil Recommendations GENERAL INFORMATION Oils The SAE (Society of Automotive Engineers) grade number indicates the viscosity of the engine oil; its resistance to flow at a given temperature. The lower the SAE grade number, the lighter the oil. For example, the mono-grade oils begin with SAE 5 weight, which is a thin, light oil, and continue in viscosity up to SAE 80 or 90 weight, which are heavy gear lubricants. These oils are also known as "straight weight'', meaning they are of a single viscosity, and do not vary with engine temperature. Fig. 1: Look for the API oil identification label when choosing your engine oil
Fig. 2: Before installing a new oil filter, coat the rubber gasket with clean oil
Multi-viscosity oils offer the important advantage of being adaptable to temperature extremes. These oils have designations such as 10W-40, 20W-50, etc. The "10W-40'' means that in winter (the "W'' in the designation) the oil acts like a thin 10 weight oil, allowing the engine to spin easily when cold and offering rapid lubrication. Once the engine has warmed up, however, the oil acts like a straight 40 weight, maintaining good lubrication and protection for the engine's internal components. A 20W-50 oil would therefore be slightly heavier than and not as ideal in cold weather as the 10W-40, but would offer better protection at higher rpm and temperatures because when warm it acts like a 50 weight oil. Whichever oil viscosity you choose when changing the oil, make sure you are anticipating the temperatures your engine will be operating in until the oil is changed again. Refer to the oil viscosity chart for oil recommendations according to temperature. The API (American Petroleum Institute) designation indicates the classification of engine oil used under certain given operating conditions. Only oils designated for use "Service SG'' or greater should be used. Oils of the SG type perform a variety of functions inside the engine in addition to the basic function as a lubricant. Through a balanced system of metallic detergents and polymeric dispersants, the oil prevents the formation of high and low temperature deposits and also keeps sludge and particles of dirt in suspension. Acids, particularly sulfuric acid, as well as other by-products of combustion, are neutralized. Both the SAE grade number and the API designation bottle be found on the oil can. For recommended oil viscosities, refer to the chart. Fig. 3: Viscosity chart
SYNTHETIC OIL There are many excellent synthetic oils currently available that can provide better gas mileage, longer service life, and in some cases better engine protection. These benefits do not come without a few hitches, however; the main one being the price of synthetic oils, which is three or four times the price per quart of conventional oil. Synthetic oil is not for every truck and every type of driving, so you should consider your engine's condition and your type of driving. Also, check your truck's warranty conditions regarding the use of synthetic oils. Both brand new engines and older, high mileage engines are often the wrong candidates for synthetic oil. A synthetic oil can be so slippery that they can prevent the proper break-in of new engines; most manufacturers recommend that you wait until the engine is properly broken in 3000 miles (4830 km) before using synthetic oil. Older engines with wear have a different problem with synthetics: they leak more oil as they age. Slippery synthetic oils get past worn parts easily. If your truck is leaking oil past old seals you'll most probably have a much greater leak problem with synthetics. Consider your type of driving. If most of your accumulated mileage is high speed, highway type driving, the more expensive synthetic oils may be a benefit. Extended highway driving gives the engine a chance to warm up, accumulating less acids in the oil and putting less stress on the engine over the long run. Trucks with synthetic oils may show increased fuel economy in highway driving, due to less internal friction. If synthetic oil is used, it should still be replaced at regular intervals as stated in the maintenance schedule. While the oil itself will last much longer than regular oil, pollutants such as soot, water and unburned fuel still accumulate within the oil. These are the damaging elements within a motor and must be drained regularly to prevent damage. Trucks used under harder circumstances, such as stop-and-go, city type driving, short trips, or extended idling, should be serviced more frequently. For the engines in these trucks, the much greater cost of synthetic or fuel-efficient oils may not be worth the investment. Internal wear increases much quicker on these trucks, causing greater oil consumption and leakage.

It should be written on your oil fill cap. I would go with 5w-30

HI,
10W-30 would give the best performance for economy.
10W-40 may be used if your engine has trouble keeping the lighter oil in it.
Best of Luck,
Wayne