Installed Bosch Platinum WR7DP plugs in our 93 240 at 62,000 miles. Car starts quickly and accelerates smoothly. Although, I did not feel a noticable difference in performance over the Bosch copper plugs I installed previously. I change our plugs annually. I don't believe I am wasting our money. At $1.79 a plug, plus tax it averages out to .63 a month. I believe it is a better plug.

Also using Mobil 1 oil in our 93 240 and 98 S90. Previously, we always used conventional oil in our american cars. Three cars we put near 200,000 miles on each of them - 1973 Beetle, 1977 Olds Cutless and a 1988 Mercury Cougar. Oil and filter was always changed every 3000 miles. Although we never experienced an engine failure, I believe the synthetic oil provides superior cooling and lubricating. I haven't decided though if I should change the oil every 5000 miles now or stick with every 3000 miles. Oil still gets dirty whether it is synthetic or not. Using synthetic oil is not that much more expensive then conventional oil. Considering I buy conventional oil at $1.14 a quart and Mobil 1 at $3.95 a quart, changing oil every 3000 miles cost me $55.00 dollars a year or $2.50 month. I use Volvo filters, but will be switching to Mann filters because they cost half the price for the same quality.

I believe for a total of $3.00 a month for plugs and oil is very little for something you care a lot so much for.