I was at the local autoparts store to buy mobil 1 syntec and he told me they don't make 20-50syntec any more the only grade like that is 15w-40syntec if this is true is this still ok to run??

Mobil 1 makes 15w-50, Castrol makes syntec. Either way 15w-40 would be OK to run for stock power.

ttt

Mobil does make a 15w-50 in their "Mobil 1" line, which is a synthetic oil. As far as I know, they do not offer Mobil 1 in a 20w-50 weight.
Castrol is the manufacturer of "Syntec", which is also a synthetic oil. I am not sure of what weights of oil are offered by them, as I do not use this oil.
Mercruiser recommends SAE 40W in their production engines.
I would say that any weight with a XXw-50 rating would be fine to use in a performance oriented marine engine.

Mobil 1 makes 3 major weights of oil. 5w-30 has a black cap on the bottle. 10w-30 has a green cap on the bottle, and 15w-50 has a red cap on the bottle.

Now for cold climates they also have: 0w-20, 0w-30, and 0w-40

Roby

They also make Mobil 1 in 0-30 and 0-40. I have 7 qts of each in the truck as that is what Saab reccommends to use in the new engines.

Jon

They make a 20w50 conventional...I have used it many times. Blue bottle with a red cap. Sure is cheaper than the synthetic 15w50 I am running now That is a silver bottle with a red cap

Castrol makes a 20w50 synthetic that alot of guys I know use in their boats....

I have been running Mobil 1 - 15w50 for along time in Big Power applications and it has been fine for me. I have also used a couple of conventional oils 20w50 and 15w40 that have been very good. Mobil 1 is high doller though as is their gear lube which I also use religiously.

Hope the info is helpfull.

Sean