It's time to drain and refill the rear axle on my 1987 Mitsubishi Montero, which has the factory limited slip differential. I went to the local auto parts store for the appropriate SAE90 GL5 oil, where I spotted Castrol Limited Slip 80W-90 and thought "great! No additive pack to put in". When I got the stuff home and opened the bottle up, I saw that it had neither the color nor the distinctive smell of a gear oil with LSD additive. Yet, the Castrol product sheet on their website says it's suitable for "service fill of limited slip differentials". I do have a bottle of LSD additive, and the question is: should I trust Castrol's product sheet and risk damaging the clutch pack by running it straight, or do I put in the additive and risk having too much friction modifier for the application? Or is there a third option?