As a native speaker, I'd say "saya" is the formal pronoun, and "aku" the informal one. Forms and documents often use "saya", while "aku" is more commonly used in daily speech.
How does mau compare to ingin? Using Mango Languages and they have ingin as meaning "to want"
They're pretty much synonyms, though in casual language you're likelier to encounter "mau".
How do I recognise if it means "I eat an apple" or "I eat apples"? How is the plural of apple defined?