Best spark plug for non-turbo 70 series?

[Dovetail]

Hello all,
This topic has been adressed before by others, but I think that my results from 3 separate cars runs counter to what is advised. Our family has two 2000 V70s (191k, 133k) and 1 1998 S70 (133k), all non-turbo. Experience with Volvo plugs (Volvo labeled plugs which have multiple electrodes placed circumferentially, not single gap old style plugs) on 3 separate occasions resulted in only transient improvements in MPG, with MPG falling after about 5000 miles (in one case poor mileage from the start which improved with Bosch platinum plus plugs which were subsequently run for >50k miles owing to excellent mpg). I have also had poor MPG develop with the use of single gap electodes (gapped to 0.028), and other Bosch Platinum plus plugs. Interestingly, the last set of Bosch Platinum plus plugs were run for 15k miles, with poor mileage for the last10k miles, and when the plugs were removed there was brown discoloration of the porcelain insulator around several of the plugs indicating arcing from the central core. So please share your experience with plugs. Perhaps by removing single gap plugs regularly (say every 5k miles and re-gapping would be the best way to go? MPG variance was at peak efficiency22-24 mpg, down to 19 mpg when not efficient. Usually regardless of brand, Volvo or Bosch there is transient efficiency gains which are unfortunately not consistently maintained. Should I consider another brand (ie NGK)? Thanks for your input.
William

[rspi]

OEM. Get what the manual suggest, not what you prefer. There are several other factors that may affect your mileage ranging from tire pressure to wires to ambient air temp to driving style.

[jblackburn]

Volvos HATE platinum plugs.

I refuse to pay for OEM plugs for $50. Just seems dumb to me. These cars love the Bosch Super Copper plugs (you can buy them at Autozone). Gap to .028" for turbos, .030" for N/A.