If apartheid had managed to survive through the 1990s, I think the system would have been more firmly entrenched today than it was in the late 1980s. Firstly, South Africa's economy is dependent on the export of natural (especially mineral) resources. Between 1983-2003 these experienced a 20 year slump in which they were low and hurt commodity exporting nations. With the rise in commodity prices, this would have only helped the regime (much as it has kept Chavez in power in Venezuela). China and to a lesser extent Japan and Korea's appetite for raw materials has caused an economic boom in Africa for the first time since the 1960s.
Secondly, the balance of power in the world is changing, and with a more multi-polar world emerging, this would have only benefited the regime. The reemergence of Russia as an authoritarian power under Putin and the growing power of China (also authoritarian), two former foes could have helped apartheid South Africa.
During the final days of apartheid in 1989-1990, SA was starting to cozy up to the USSR a bit. There were many in the country who advocated taking in a large number of Eastern European immigrants. By accepting a large enough number (say 500,000), this would have helped the country in a few ways. Firstly, it would boost the white population with "petits blancs" who were often more supportive of apartheid than the English-speaking community. Secondly, it would give South Africa some leverage in diplomatic relations with the host country. South Africa would be willing to take in engineers and highly skilled people from the former Eastern bloc to beef up the armaments industry. In OTL they had around 16,000 Poles in the country by 1990, so this is one community that could have grown during the economic turmoil of the 90s in Eastern Europe.
With the collapse of many of the Eastern bloc economies, South Africa could have acquired weapons and technology from ex-Soviet states even through smuggling and bribery. If a large enough Russian community developed in South Africa (say 250,000 Russians by today), it would not be inconceivable that the Russian government would try to be soft on South Africa for the sake of protecting their compatriots. Also, Putin's Russia has been hesitant in criticizing other regimes due to their internal policies or lack of democratic rights.
China would have become more important to South Africa if apartheid had survived. The Chinese have generally adopted a foreign policy of turning a blind eye to the internal problems of a country, for the sake of trade. Just look at their track record with Iran and Sudan. With its vast amounts of natural resources, the Chinese would simply want to do business. They may not establish official ties, but I'm sure their businessmen would be treated as "honorary whites" just as Japanese and Taiwanese were during the apartheid era.
In 2011, trade between China and South Africa totaled $45 billion, in 1992 trade between the two countries was just under $15 million. China, is today South Africa's single largest trading partner, yet Japan still accounts for a large part of the trade and investment in South Africa, much of this dating from the 1980s when the Japanese sought to displace European and American firms in the country.
As for oil, perhaps South Africa would be one of the few countries willing to import Iranian oil today. Iran which is strapped for foreign reserves might have been willing to trade oil for other resources such as gold. The irony of this would be that relations between Israel and South Africa could have been jeopardized. However, China is often against sanctions on Iran, yet has a cozy relationship with Israel when it comes to transferring weapons technology.