10w30 + 20w50 = 10w50 ?

More like 15W40 and that is only a guess.

I am curious. What are you trying to accomplish? In what environment are you trying to protect your engine? DO you need 10w50 oil features or is this an experiment?

I use 20w50 during the summer. But, I need something 10w- during winter cold start, while maintaining -w50 when engine is hot. This winter, I used 15w40 (diesel), and it seemed okey. But, I'm wondering if I can just mix two bottle and get something resembling 10w50...

Maybe my Michigan climate is different than yours, but at least you know we've got cold weather too.

I've used both 20w50 year round and 5w50 syn year round. The only difference I saw was that the syn started easier on those sub-zero (F) days.

Tim Wohlford '89 Golf

Note: you should first stay within the specifications recommended by the manufacturer. If you have a diesel make sure all the oil you use meets the diesel specs for your 86 Jeta (likely just "C" If gasoline make sure it is rated for gasoline "S"

You should be able to find 10W50 synthetic.

Mine is gasoline. Unfortunately, I don't have original owner's manual. Local dealer says they use 15w40 year round. But, once hot, my car seems to run better with -w50 than -w40.

Stop, don't do this.

2) There is no reason NOT to run synthetic. If your seals are fine (you're not leaking oil, or burning it), synthetic will still offer significant benefits over conventional oils. (And if your leaking or burning, overhaul the engine, and run synthetic.)