Gee, feller, what's the problem? I'm from the old school, and i always just use good old Dino oil in all of mine! SAE 30 keeps mine going!
Since you have evidently bought a used machine, you'd might as well not bother with those exotic hyped up oils, and just go with 30W!
My Snapper rear engine rider mower always had 30W put in it for oil changes, and it lasted 13 years, but didn't blow up, the thing was just tired!

Rusty is right but you will find many length threads on the subject. To satisfy yourself, just go to the B&S site and look it up.

Walt Conner

Jim,

That B&S Vanguard is a commercial grade engine and should be treated with respect rather than lumped in with the multitude of flat heads and one-lungers under the hood of whatever...

Here's a link to where you can view and download the operator's manual for your GT235... click here

With the serial # from your GT235 you can choose exactly the correct manual.

I'd follow the viscosity recommendations in the manual for peace of mind.

Not knowing the service history of your new used GT I'd stick with a quality brand of dino oil and follow the prescribed maintenance JD recommends in the manual.

I use SAE 30 in all small/ medium block Vangards for summer use. Deere's lube chart is all over the board- actually shows 5w30. Unsure what the OM says. The van is a nice enough engine, but nothing special enough to warrant anything you don't want to pay for. Synthetic is certainly fine in a straight or 30 weight multigrade. On customer units that warrant a 10W30- I use a syn blend.

Mobil -1 synthetic is used in 4 of my engines , 10w-30 in my Deere, been using synthetic for over 35 years. You can use synthetic if the engine new, not reccomended for an older engine or if it has been on a conventional lube diet.... The one Deere I have used conventional for 17 years so I stayed with it , the previous owner used 30hd so that is what I used.