Spanish subjunctive question

[Jaleel10]

No utilice medios de transporte que usen gasolina

What is the general rule for it's usage here. Most people say subject change but that really doesn't help me.

[Dragon27]

Spanish subjunctive doesn't have this kind of strict grammar rules for it's general usage (although one can memorize some cases in which it definitely should be used). Subjunctive in Spanish expresses the attitude of the speaker with respect to what the speaker is saying.
I would personally classify this case as an example of this usage:

Para hablar de las características de personas o cosas desconocidas, no concretas, hipotéticas:

Quiero comprar un departamento que tenga tres dormitorios.
Necesito una secretaria que sepa inglés, alemán, francés y español.
Busco amigos que sean sinceros, simpáticos y que hablen español.

http://www.elsubjuntivo.com/usos.html

[tarvos]

I think that's not the reason. The reason is that it is conditional and dependent on the cars that use gasoline and only those cars.

Cfr. Siempre que los medios de transporte usen gasolina

And with querer and necesitar you always need the subjunctive...

[Dragon27]

I don't know. I don't have that feel of the Spanish language yet.
Another good resource:
http://uni.canuelo.net/wp-content/uploa ... os_nvo.pdf
Maybe this one?

En frases de relativo
Indicativo | Cosas conocidas | Quiero que traigas el libro que está en la mesa.
Subjuntivo | Cosas desconocidas* | Quiero que traigas el libro que esté en la mesa.
Subjuntivo | Antecedente negado o negativo | Esto no hay quien lo entienda. No hay nada que me guste. Aquí no hay quien viva.

[rfnsoares]

No utilice medios de transporte que usen gasolina

What is the general rule for it's usage here. Most people say subject change but that really doesn't help me.

As a Portuguese native speaker, I always apply the Present of Subjunctive after the conjunction "que" (that/which). When I was studying Spanish and Italian grammar, I used to practice the Subjunctive with conjunction "que" (che in Italian). You can use the Future of Indicative in very few circumstances when talking about future.

Many people in Brazil apply the Present of Indicative after the conjunction "que" and, to be honest, it sounds so weird, odd to me! It hurts my ears.

[tastyonions]

I agree with Dragon27. I would say that it's because the sentence is talking about unspecified, "possible" vehicles that use gasoline, rather than referring to ones that we know (through context) are present or have already been referenced. There's a bit of a wrinkle in that the sentence is imperative ("no utilice"), but you can see a similar phenomenon when you talk about something you are looking for:

Estoy buscando un árbol que me dé sombra.
Está buscando un hotel que tenga baño limpio.