If you run into warranty issues and you use oil outside of what is specified for the 2003 you better have a good explanation why you did not use the spec'd oil. It explicit says in the manual to use 5-30 or 10-30 if 5-30 is not available. The 5-30 covers to 100 degrees. You could go with 10-30 and still be within their "rules". The heavier weights will not hurt the motor its just that you will be at a disadvantage if something fails and they want documented proof of oil changes.
Direct from Toyota:
All Toyota vehicles come from the factory with natural petroleum-based engine oil. Toyota is currently recommending API1 grade SJ or SL, or ILSAC2 multigrade petroleum-based engine oil3 for our current model year vehicles. In moderate climates, this oil should have a Society of Automotive Engineers (SAE) viscosity rating of 5W-30. In our high mileage tests with preventative maintenance performed at the recommended intervals, the recommended natural petroleum-based oil has provided excellent service.
If you decide to use synthetic oil for the engine, it is best not to switch until the first scheduled oil change. Synthetic oil should meet or exceed the specifications provided in your Owner's Manual. Even if synthetic oil is used, we do not recommend a longer oil change interval. Also, once synthetic oil is used, you should keep using it and not switch back and forth with natural petroleum-based oil.
The Toyota New Vehicle Limited Warranty does not cover any damage to the engine resulting from the use of engine oil that is defective, or that does not meet the specifications provided in your Owner's Manual, regardless of whether the oil is natural or synthetic. You can protect yourself against this possibility by having your maintenance performed by a factory-authorized Toyota dealership.
Daily Drives:
-2003 Toyota Tacoma PreRunner XtraCab, 2.7 Liter , Mobil1 Synthetic SS 5W-30.
ODO 7200 Miles.
-1995 Toyota 4-Runner 3.0 V6, Mobil1 Synthetic SS 10W-30.
ODO 83300 Miles.
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