Is 20W-50 too heavy for my 340?

I have mentioned this previously, but I formulated Delo 400 for Chevron for 6 years and did additive development for 12 years. I run Delo 400 SAE 15W-40 HDDO in my older vehicles with perfect success. HDDO Heavy Duty Diesel engine Oils) have the same Zinc content as older oils and work fine with flat tappet cams. Ran my '66 Hemi, '67 383 Cuda and '68 340 on this oil for decades. The HDDO has much more additive than conventional oils and really keeps the engine clean plus protects the cam from wear. Other similar products are Shell Rotella T and Mobil Delvac 1300. All will work. You can special order SAE 10W-30 which is perfect for older street engines. I do 3,000 mile drains to keep the oil fresh. Doesn't hurt to have clean oil.