By Sarah Marsh

HAVANA (Reuters) - When they are not tending to international affairs, diplomats based in Havana can be found these days stewing in interminable queues at gas stations and concocting ways to increase the octane in fuel as Cuba's premium gasoline shortage takes its toll.

Cuba sent around an internal memo last week advising that it would restrict sales of high-octane, so-called "special fuel" in April. That is not an issue for most Cuban drivers, whose vintage American cars and Soviet-era Ladas use regular fuel.

But it is for the embassies that use modern cars whose engines could be damaged by the fuel at most Havana gas stations. So the diplomats are taking a leaf out of the book of Cubans, used to such shortages, and becoming resourceful.

Given the U.S. trade embargo, Cubans have for decades had to invent new ways to keep their cars on the road, replacing original engines with Russian ones and using homemade parts.

"I bought octane booster, and the embassy has bought lubricants, meant to help the motor deal with rubbish gasoline," said one north European diplomat, who got a relative to bring the booster in his luggage given it is unavailable in Cuba.

"At the moment we are using the car that runs on diesel, so we can 'survive'," said an Eastern European diplomat.

Cuba has not announced the measure officially yet. According to the memo, "the special fuel remaining in stock at gas stations from April will only be sold in cash and to tourists until the inventory is depleted".

"It's very serious. I have already suspended a trip to Santiago de Cuba for fear of lack of gas," said one Latin American diplomat, adding that it seemed like the problem would last. "Diplomats are very worried."

Some embassies in Havana have people scouting out which stations still have some higher-octane fuel and are sending around regular updates to staff. One gas station worker said they were getting small deliveries of fuel each day still.

The embassies are also advising people to carpool or use the diplomatic shuttle.

Meanwhile the European Union has requested from the ministry of foreign affairs that one or more service centers be set aside for diplomats with special gas, according to a European diplomat.

Cuba has become increasingly reliant on its socialist ally Venezuela for refined oil products but the latter has faced its own fuel shortage in recent weeks.

Meanwhile, the Communist-ruled island cannot easily replace subsidized Venezuelan supplies as it is strapped for cash.

Although the memo referred to April, it is not clear how long the shortage will last. Cubans joke that once something disappears in Cuba, it is never to return, referring to products that have disappeared from their ration book like cigarettes, beef and condensed milk.

The Peugeot dealership in Havana has sent its clients lists of technical tips how to protect their motors while using lower-grade gasoline, including more frequent maintenance and ensuring vehicles at running at optimum temperature before driving.

The shortage is also impacting others using modern cars such as taxi drivers, tourists and workers at joint ventures.

(Additional reporting by Marc Frank; Editing by Lisa Shumaker)