When in doubt, use 125°C rated TXL, GXL, SXL, or SGX wire insulation for engine compartment and general wiring.  The only difference between these is the thickness of the insulation and cost, with the thicker being the more expensive. They are all insulated with crosslinked polyethylene, which is almost as heat resistant as teflon, but cheaper and much tougher.
The wire available in your local automotive or hardware store is insulated with PolyVinyl Chloride (PVC). PVC is OK for cool areas, but NOT under the hood. The PVC will melt at temperatures that don't affect the HDPE! For more detail, see wire tables and amps vs wire size.

TXL is the wire used by vehicle manufacturers for most applications, and is the thinnest, lightest, and lowest cost of these insulations. It is also tough and long lasting.
To order 18 to 12 Gauge TXL Wire

GXL is an intermediate insulation,with thickness and cost between TXL and SXL.  

To order 18 to 14 Gauge GXL Wire

SXL is the second thickest of the crosslinked polyethylene insulations, and suggested for use in heavier gauges, or where additional protection from possible mechanical damage is needed.

To order 12 to 6 Gauge SXL Cable

SGX is the thickest of the crosslinked polyethylene insulations. SGX is particularly recommended for battery cable, because of the rather spectacular consequences of a shorted battery cable.

To order 4 to 1/0 Gauge SGX Cable

NOTE: SAE J1128 GPT is a PVC insulated wire rated at 85°C used for general chassis wiring, and should NOT be used in the engine compartment of your car according to the SAE J1128 spec. It's not as tough or heat resistant as TXL or GXL insulation, although it is significantly cheaper.
This is the wire found in retail stores.

For more info on TXL, GXL, or SXL/SGX Wire,