and is usually said to be 90% intelligible (for the formal/standard language) since it came from the same root.
But from what I observe nowadays, younger generation in Indonesia will have more difficulty in understanding Malay because there are many vocabularies that is now become rarely used by them. And with more Indonesians understanding English, they just won't bother to think about trying to understand Malay when they happen to meet Malaysians.
Side story: As I am Chinese descent with ancestry from Fujian Province, I speak Hokkien. And I found out that even Hokkien speakers in Indonesia has different variations, where sometimes the difference in the vocabulary can be quite big. The Medan Hokkien (the one I spoke) is mostly intelligible with Penang Hokkien in Malaysia, which means it's quite isolated compared to other variants. My father once arranged to meet a Singaporean businessman and my father thought that as they both can speak Hokkien it is alright (he barely understands English or Mandarin). When they start discussing, it turns out my father doesn't totally understand what the Singaporean was saying. Luckily I was there, and suddenly became the middleman between them lol