depending on your weather situation and driving condition.

If you are in SoCali or Florida type of weather where it is hot, I would run heavier grade oil ie: 10w30 or 10w40 all year round.

If in COld conditions then go with the 5w grade. If any one ever bullshit you to use 20W grade, you should walk away from that store. I had that happen to me once. took my car to Walmart, and saw them about to pouring some "Tech 2000" 20w grade oil in my car. Good thing I caught them in time, and had a good argument witht he service counter guy.

I never trust any mechanic anymore ie: jiffy lube, 10 minute lube, or where ever other places there are as they only focus is on getting you car back to you ASAP. Sometime they do not fully drain all old oil out of you car. Not to mention they replace with oil that is already outlived their shelf life.

You best bet....do it yourself. that way you'll be able to answer all of your stated questions yourself.

I use Purolator filter and Mobile 1 synthetic (green cap) on my 420a and 4g63 every 3500 miles. It is not hard to do. Just a 17mm wrench, a suitable container, some towels, and 30 minutes is all you need. whle waiting for the oil to drain out, you can always check your tranny fluid, plugs, air filter, belts, hoses, turbo (if you have one), or even just chill and take a glup of beer.

Thinking about switching to Castrol synthetic though. I like the name and bottle...:P