I change my oil every 3K miles, always. I also change tranny fluid every 10 - 12K miles (anually). Now, is there any real advantage to use syn with these intervals? I don't drive the cars (2 97 850 GLT Turbos) hard. I have changed the air filters to Knb's. I always run 91 octane..... I put BG 44K in the gas twice a year, and add a BG tranny conditioner each change. It costs, $43 to do a tranny flush, and $16 for an oil change. The BG44K is $40/year. What would the advantage be to go to syn. One car has 40K and the other 80K. These costs are per car. Would going to syn be kind of getting to the point of diminishing returns?
So my anual costs for this is:

Oil 3 X $16 = $48
Tranny 1 X $43 = $43
BG additives 2 X $20 = $40
Annual costs $131

X 2 cars $262

Thanks....