Well, sounds like most of you are going to jump all over me. But I crunched the numbers and went for the synthetic (polyester) pine straw. I put it over weed cloth in my flowerbeds in 2004 and it still looks great. It and the weedcloth have cut my weeding time by about 98%. The artificial straw contains no molds, diseases nor insects. My beds are on high ground, and the straw stays put. There is one area that gets a lot of leaves. For that, I take the straw out on the lawn on a lovely day, sit on my rolling garden wagon and shake the leaves out, bagging the cleaned straw as I go. Then I blow any remaining debris off the weed cloth and replace the straw. I understand that they now have a glue that one sprays over newly installed texstraw so that you can use a blower to get the leaves off without disturbing the straw. That might work if your planting were all perennials and you weren't planning on new plants for a while. When I want to add a plant, I simply remove the straw from that area, cut an x in the weed cloth, plant, and replace the straw around the new plant. No problems. My spring bulbs come right up as usual.

You can also get Royal Purple synthetic 10W30 for motorcycles (air-cooled service). I've been using that for six years in my Honda GSV 190 commercial mower engine. Overkill, but its a great engine that I want to keep a long time. Even though the oil is $12/qt., its still only $8 per yearly change. But, to the question, a 10W30 synthetic blend meeting the latest SAE service standards is certainly more than good enough for a mower engine (where 10W30 is allowed by the manufacturer). In a hot climate I would use SAE 30 HD.

for a tiled shower? I'm freaking out a little bit because that's what the contractor used on my shower, which isn't complete yet (still have to grout it) and all I can find about this product is that it's used under roofing shingles.

I do not understand the question What exactly are you doing ? Usually decking is either wood or wood look and if you want something not "wood" you go with something like Duradeck.