Hi, I went from Castrol Syntrax 75W90 to Swepco. You could class the Castrol as being the correct spec of oil, whereas Swepco is a thicker lubricant, recommended by enthusiasts for our gearbox. My car is a 220T with LSD and external oil cooler factory options that's done 75k. Second gear synchro is worn. Driving away cold the Castrol requires less force to affect a gearchange. After driving for 10-15 mins the Swepco warms up. The Swepco gives a smoother and slightly lighter change once warmed through than on Castrol. Its a heavy change down into second. Since switching to Swepco I've always preferred to double declutch my changes until the gearbox oil has warmed through as it is a lot lighter thereafter. I find that changing up gear in excess of 3000rpm gives a lighter change than my pootling around mode where i change up at 2000rpm. In fact above 3000rpm I really like the weight and feel of my cars gear change. I bought the Swepco from Berlyn Servicees, many years ago, so can't advise on price.
