WHY do Chinese communist leaders live so long? Because they drink a tonic called the Chinese Wonderful Prescription, according to the Hongkong agent for the product. The Washer (International) Co claims the ''outstanding and superior'' elixir not only improves health, fitness and mental alertness but boosts sexual performance. One satisfied 60-year-old says the miracle brew gave him his first erection in 10 years - and his wife is delighted. According to Washer: ''The medical herbs used for making CWP are rare and expensive. That's why the Medical Research Academy of China can only make a small quantity for government leaders, ensuring their vigour and long lives.'' But it seems the vulgar need for cash has forced the academy to look for clients beyond the confines of the leadership. In a mail order marketing spiel, Washer managing director Mr Ramon Kung said: ''To make some profits, CWP is now being made on a smallscale for marketing.'' The mail order drive seems to have been aimed at Hongkong executives, who are being offered the CWP prescription capsule A and the prescription suppository for a mere US$77 (HK$595) per pack. ''The average life span for those family members who insist on taking CWP is 91 years old,'' Mr Kung said. They are not only able to preserve and enhance their sexual prowess but are offered the chance ''to build up physique, avoid fatigue and enhance mental ability''. They are able ''to put up with coldness as well as hotness''. At the same time, ''old men's spots decrease'' and the customer is assured of ''being in full vigour''. But CWP is not for everyone. Mr Kung sternly warns would-be customers the supply is limited.