c for 12 hours the chlorine content in the aluminum trichloride supported catalyst is assayed to be 7 77 wt by volhard titration to 250 ml high pressure polymerizer is added 80 ml of n hexane 8 g of catalyst obtained as above 30 ml of 1 decene and 0 159 ml of water as promoter 1 decene oligomerization is carried out at 80 u00b0 c under pressure of 1 0 mpa for 5 hours then the reaction is stopped the catalyst is filtered n hexane solvent is removed by atmospheric distillation unreacted monomer 1 decene and part of dimers are removed by reduced pressure distillation 1 decene oligomer is finally obtained the conversion rate of 1 decene is 57 8 wt the oligomer has a kinematic viscosity at 40 u00b0 c of 19 21 mm 2 s a kinematic viscosity at 100 u00b0 c of 6 69 mm 2 s and a viscosity index of 200 61 aluminum trichloride supported catalyst is prepared the same as example 1 except that the supported time for aluminum trichloride lasts for 12 hours the chlorine content in the aluminum trichloride supported catalyst is assayed to be 7 92 wt by volhard titration to 250 ml high pressure polymerizer is added 80 ml of n hexane 8 g of catalyst obtained as above 30 ml of 1 decene and 0 16 ml of water as promoter 1 decene oligomerization is carried out at 80 u00b0 c under pressure of 1 0 mpa for 5 hours then the reaction is stopped the catalyst is filtered n hexane solvent is removed by atmospheric distillation.