TECHNOLOGY FOCUS - ORGANIC CHEMISTRY - Preferred Starting Materials: At least 90 wt% of the starting olefins contain units of formula -(CH2)n- where n is 1-5 preferably, at least 90% comprises unbranched alpha-olefins and 2-methyl-alpha-olefins. Most preferred are 8-12C olefins, preferably at least 90 wt% 1-decene or 1-dodecene. Preferred Activators: (A1) comprises 1-10C alkanols and (A2) comprises dialkyl ethers with at least 5 carbons. Oligomerization may be carried out in presence of 2-20 wt% chloro-hydrocarbon, and/or compound A2 with a mol ratio of A1 to A2 = (5:1)-(1:20), the average number of carbon atoms in A1 and A2 (calculated from mol fractions) being 4-7 and the preferred A1 being a 2-6C alkanol. The type and quantity of A1 (and optionally that of A2 and/or chloro-hydrocarbon) are chosen so that phase separation gives phase 1 with a high BF3 concentration and phase 2 with a low BF3 concentration, the solubility of BF3 in phase 2 being less than 1 wt%; in this case, A1 is water, methanol or hydrogen fluoride and the other compound (b) is a 5-10C dialkyl ether (A2) (with a mol ratio of A1 to A2 = (1:2)-(1:20)), or A2 is absent and A1 is ethanol, isopropanol or n-propanol. Preferred Method: Oligomerization at -20 to +40degreesC with a BF3 concentration below the saturation concentration at reaction temperature under a BF3 pressure of 1 bar, preferably with 0.5-2 wt% BF3 in the mixture. The product obtained (I) is then subjected to catalytic hydrogenation. EXAMPLE - A mixture of 5 mmols n-pentanol and 5 mmols dihexyl ether was cooled to -10degreesC, treated with 10 mmols boron trifluoride and 140 g 1-decene, warmed to 30degreesC, treated with a further 10 mmols BF3, kept at 30degreesC for 2 hours and mixed with 200 ml ice water. The organic phase was washed with 2 x 200 ml water and worked up by evaporation at 150degreesC and 0.1 mbar to give a product with a dimer content of less than 2 wt%, a viscosity of 5.12 mm2/sec at 100degreesC and a viscosity index of 136. The yield was 87% (11% if no dihexyl ether was used).