I want to change the gear oil of my truck (manual transmission 91 Toyota pickup) and it calls for SAE 75W-90. I live in So. California. Can I substitute SAE 80W-90 instead? The autoparts store said it should be fine. Just want to get a second opinion. Thanks.

Yes that should be fine...
I have a 1993 & use 80 - 90 in my 389,000 mile Toyota

For superb shifting, better gas mileage, and wear protection, I would use Mobil 1 75W-90 gear oil:

Mobil 1 Synthetic Gear Lube LS 75W-90

There's so much friction in transmissions and differentials, that the greatest "bang for the buck" comes from using synthetics in these applications. When I used to live in the Santa Monica mountains, just north of Malibu, I switched to synthetics in my transmission and differential to increase horsepower and mountain climbing ability.

Whichever you decide to use, change the transmission's gear oil after it reaches normal operating temperature. This provides a complete and fast drain.