The following quote was taken from a Russian Online Hoya Catalog:

“Flowers collected in the round umbrellas. Corolla yellow color, smooth on the outside, with a delicate hairs in, 0.7 – 0.8 cm in diameter; . white crown flower Hoya quinquenerviacan be easily distinguished from Hoya merrillii (also abundantly covered with leaves species with yellow flowers) in Hoya quinquenervia corolla lobes horizontal, while Hoya merrillii corolla lobes are raised at the outer ends. There are many other differences, but it is – the main thing that is easy to see.”