15w50! Where do you drive; Texas Motor Speedway? Seriously 15w50 is generally considered by most oil experts as way too thick for a modern street driven car. An excellent resource on oil and filter testing is BITOG;La-Z-Link
The Probe could possibly benefit from such a thick oil due to excessive wear; but I think a better choice for the TT ia a quality synthetic in the range of 0w30; 5or10w30 (Nissan reccomended) or at most a 0w40. I am currently using German Castrol 0w30 (sold at AutoZone). At BITOG there is a dedicated forum to German Castrol with a lot of believers; Check it out.