It is true, for example Viper hoses state the following "SUITABLE FOR: Any Water, Air, Coolant, oil breather applications. Not suitable dedicated fuel, oil or brake fluid applications. " and thats £21 a metre!!

I would imagine you could get blocked oilways, blocked lifters, and maybe a clogged turbo,etc...! But what we can say for 100% is that the oil will not 'behave' as it should because of being diluted with rubber!