The colder it gets the thinner your fork oil needs to be. 2.5 wt oil is as thin as it gets,but the price is a crime.
Silkolene's advanced racing suspension fluids are designed for the latest competition suspension systems. They feature low friction synthetics, anti-foam and wear-resistant additives to ensure predictable suspension performance, long seal life and silky smooth action. They are used extensively by factory teams and suspension specialists in road racing and rallying competition. The oil provides exceptional performance in air, nitrogen and argon filled systems, and has a high viscosity index (VI), which eliminates the 'fade' experienced when shocks are required to operate at high temperatures. Available in a variety of grades which can be individually blended to give an expertly fine-tuned suspension response.